diff --git a/doc/sphinx/cxx-guide/thermodemo.cpp b/doc/sphinx/cxx-guide/thermodemo.cpp
index ca3ed9013..2a1b3c65c 100644
--- a/doc/sphinx/cxx-guide/thermodemo.cpp
+++ b/doc/sphinx/cxx-guide/thermodemo.cpp
@@ -2,7 +2,7 @@
 
 using namespace Cantera;
 
-void thermo_demo(std::string file, std::string phase)
+void thermo_demo(const std::string& file, const std::string& phase)
 {
     ThermoPhase* gas = newPhase(file, phase);
     gas->setState_TPX(1500.0, 2.0*OneAtm, "O2:1.0, H2:3.0, AR:1.0");
diff --git a/include/cantera/Edge.h b/include/cantera/Edge.h
index b5a42e4e8..cbf68d49c 100644
--- a/include/cantera/Edge.h
+++ b/include/cantera/Edge.h
@@ -14,7 +14,7 @@ class Edge :
     public EdgePhase, public EdgeKinetics
 {
 public:
-    Edge(std::string infile, std::string id, std::vector<ThermoPhase*> phases)
+    Edge(const std::string& infile, std::string id, std::vector<ThermoPhase*> phases)
         : m_ok(false), m_r(0) {
 
         m_r = get_XML_File(infile);
diff --git a/include/cantera/IdealGasMix.h b/include/cantera/IdealGasMix.h
index acd9febe7..82ab1529f 100644
--- a/include/cantera/IdealGasMix.h
+++ b/include/cantera/IdealGasMix.h
@@ -19,7 +19,7 @@ public:
 
     IdealGasMix() : m_ok(false), m_r(0) {}
 
-    IdealGasMix(std::string infile, std::string id="") :
+    IdealGasMix(const std::string& infile, std::string id="") :
         m_ok(false), m_r(0) {
 
         m_r = get_XML_File(infile);
diff --git a/include/cantera/IncompressibleSolid.h b/include/cantera/IncompressibleSolid.h
index cd0d8ae29..652b007e8 100644
--- a/include/cantera/IncompressibleSolid.h
+++ b/include/cantera/IncompressibleSolid.h
@@ -12,7 +12,7 @@ namespace Cantera
 class IncompressibleSolid : public ConstDensityThermo
 {
 public:
-    IncompressibleSolid(std::string infile,
+    IncompressibleSolid(const std::string& infile,
                         std::string id="") : m_ok(false), m_r(0) {
 
         m_r = get_XML_File(infile);
diff --git a/include/cantera/Interface.h b/include/cantera/Interface.h
index 120c6423b..1280bd78b 100644
--- a/include/cantera/Interface.h
+++ b/include/cantera/Interface.h
@@ -44,7 +44,7 @@ public:
      *   the ReactingSurface class. These classes will be migrated into Cantera
      *   soon.
      */
-    Interface(std::string infile, std::string id,
+    Interface(const std::string& infile, std::string id,
               std::vector<Cantera::ThermoPhase*> otherPhases) :
         m_ok(false),
         m_r(0) {
@@ -122,7 +122,8 @@ protected:
 /*!
  *  This is the preferred method to create an Interface instance.
  */
-Interface* importInterface(std::string infile, std::string id, std::vector<Cantera::ThermoPhase*> phases)
+Interface* importInterface(const std::string& infile, const std::string& id,
+                           std::vector<Cantera::ThermoPhase*> phases)
 {
     return new Interface(infile, id, phases);
 }
diff --git a/include/cantera/Metal.h b/include/cantera/Metal.h
index 1fa35fe60..c37556fe1 100644
--- a/include/cantera/Metal.h
+++ b/include/cantera/Metal.h
@@ -12,7 +12,7 @@ namespace Cantera
 class Metal : public MetalPhase
 {
 public:
-    Metal(std::string infile, std::string id="") : m_ok(false), m_r(0) {
+    Metal(const std::string& infile, std::string id="") : m_ok(false), m_r(0) {
 
         m_r = get_XML_File(infile);
         if (id == "-") {
diff --git a/include/cantera/PureFluid.h b/include/cantera/PureFluid.h
index 9ff149587..4a0c0c05b 100644
--- a/include/cantera/PureFluid.h
+++ b/include/cantera/PureFluid.h
@@ -17,7 +17,7 @@ public:
 
     PureFluid() : m_ok(false), m_r(0) {}
 
-    PureFluid(std::string infile, std::string id="") : m_ok(false), m_r(0) {
+    PureFluid(const std::string& infile, std::string id="") : m_ok(false), m_r(0) {
 
         m_r = get_XML_File(infile);
         if (id == "-") {
@@ -29,7 +29,7 @@ public:
     }
 
 
-    PureFluid(XML_Node& root, std::string id) : m_ok(false), m_r(0) {
+    PureFluid(XML_Node& root, const std::string& id) : m_ok(false), m_r(0) {
         m_ok = buildSolutionFromXML(root, id, "phase", this, 0);
     }
 
diff --git a/include/cantera/base/XML_Writer.h b/include/cantera/base/XML_Writer.h
index 94054385b..f57edf5e3 100644
--- a/include/cantera/base/XML_Writer.h
+++ b/include/cantera/base/XML_Writer.h
@@ -26,7 +26,7 @@ public:
         return m_s;
     }
 
-    inline std::string XML_filter(std::string name) {
+    inline std::string XML_filter(const std::string& name) {
         int ns = static_cast<int>(name.size());
         std::string nm(name);
         for (int m = 0; m < ns; m++)
diff --git a/include/cantera/base/ctexceptions.h b/include/cantera/base/ctexceptions.h
index 602d0cb82..29093127d 100644
--- a/include/cantera/base/ctexceptions.h
+++ b/include/cantera/base/ctexceptions.h
@@ -78,7 +78,7 @@ public:
      *             generated.
      * @param msg  Descriptive string describing the type of error message.
      */
-    CanteraError(std::string procedure, std::string msg);
+    CanteraError(const std::string& procedure, const std::string& msg);
 
     //! Destructor for base class does nothing
     virtual ~CanteraError() throw() {};
@@ -102,7 +102,7 @@ protected:
     CanteraError() : saved_(false) {};
 
     //! Constructor used by derived classes that override getMessage()
-    CanteraError(std::string procedure);
+    explicit CanteraError(const std::string& procedure);
 
     //! The name of the procedure where the exception occurred
     std::string procedure_;
@@ -134,7 +134,7 @@ public:
      * @param sz   This is the length supplied to Cantera.
      * @param reqd This is the required length needed by Cantera
      */
-    ArraySizeError(std::string procedure, size_t sz, size_t reqd) :
+    ArraySizeError(const std::string& procedure, size_t sz, size_t reqd) :
         CanteraError(procedure), sz_(sz), reqd_(reqd) {}
 
     virtual std::string getMessage() const;
@@ -164,7 +164,7 @@ public:
      * @param mmax This is the maximum allowed value of the index. The
      *             minimum allowed value is assumed to be 0.
      */
-    IndexError(std::string func, std::string arrayName, size_t m, size_t mmax) :
+    IndexError(const std::string& func, const std::string& arrayName, size_t m, size_t mmax) :
         CanteraError(func), arrayName_(arrayName), m_(m), mmax_(mmax) {}
 
     virtual ~IndexError() throw() {};
diff --git a/include/cantera/base/ctml.h b/include/cantera/base/ctml.h
index 738ae1c03..a602008f6 100644
--- a/include/cantera/base/ctml.h
+++ b/include/cantera/base/ctml.h
@@ -619,7 +619,7 @@ void getFloats(const Cantera::XML_Node& node, std::map<std::string, double>& v,
  *   @param parent reference to the XML_Node object of the parent XML element
  *   @param name   Name of the XML child element
  */
-int getInteger(const Cantera::XML_Node& parent, std::string name);
+int getInteger(const Cantera::XML_Node& parent, const std::string& name);
 
 //!  Get a floating-point value from a child element with a defined units field
 /*!
@@ -655,8 +655,10 @@ int getInteger(const Cantera::XML_Node& parent, std::string name);
  *                 and "" , for no conversion. The default value is "",
  *                 which implies that no conversion is allowed.
  */
-doublereal getFloatDefaultUnits(const Cantera::XML_Node& parent, std::string name,
-                                std::string defaultUnits, std::string type="toSI");
+doublereal getFloatDefaultUnits(const Cantera::XML_Node& parent,
+                                const std::string& name,
+                                const std::string& defaultUnits,
+                                const std::string& type="toSI");
 
 //!  Get an optional model name from a named child node.
 /*!
diff --git a/include/cantera/base/global.h b/include/cantera/base/global.h
index b8a9e61fa..d2127a55a 100644
--- a/include/cantera/base/global.h
+++ b/include/cantera/base/global.h
@@ -53,7 +53,7 @@ std::string lastErrorMessage();
  *
  * @ingroup errorhandling
  */
-void setError(std::string r, std::string msg);
+void setError(const std::string& r, const std::string& msg);
 
 //!  Prints all of the error messages to an ostream
 /*!
@@ -172,7 +172,7 @@ void popError();
  *
  * @ingroup inputfiles
  */
-std::string findInputFile(std::string name);
+std::string findInputFile(const std::string& name);
 
 //!  Add a directory to the input file search path.
 /*!
@@ -180,7 +180,7 @@ std::string findInputFile(std::string name);
  *
  * @param dir  String name for the directory to be added to the search path
  */
-void addDirectory(std::string dir);
+void addDirectory(const std::string& dir);
 
 //@}
 
@@ -313,14 +313,14 @@ void setLogger(Logger* logwriter);
 /*!
  * @param unit String containing the units
  */
-doublereal toSI(std::string unit);
+doublereal toSI(const std::string& unit);
 
 /// Return the conversion factor to convert activation energy unit
 /// std::string 'unit' to Kelvin.
 /*!
  * @param unit  String containing the activation energy units
  */
-doublereal actEnergyToSI(std::string unit);
+doublereal actEnergyToSI(const std::string& unit);
 
 /// Return a pointer to the XML tree for a Cantera input file.
 /*!
@@ -332,13 +332,13 @@ doublereal actEnergyToSI(std::string unit);
  * @param file String containing the relative or absolute file name
  * @param debug Debug flag
  */
-XML_Node* get_XML_File(std::string file, int debug = 0);
+XML_Node* get_XML_File(const std::string& file, int debug = 0);
 
 /// Close a Cantera input file.
 /*!
  * @param file String containing the relative or absolute file name
  */
-void close_XML_File(std::string file);
+void close_XML_File(const std::string& file);
 
 #ifdef WITH_HTML_LOGS
 
@@ -376,7 +376,7 @@ void close_XML_File(std::string file);
  *  @param loglevel loglevel of the group.
  *  @ingroup HTML_logs
  */
-void beginLogGroup(std::string title, int loglevel=-99);
+void beginLogGroup(const std::string& title, int loglevel=-99);
 
 //! Add an entry to an HTML log file.
 /*!
@@ -387,7 +387,7 @@ void beginLogGroup(std::string title, int loglevel=-99);
  *
  * @ingroup HTML_logs
  */
-void addLogEntry(std::string tag, std::string value);
+void addLogEntry(const std::string& tag, const std::string& value);
 
 //! Add an entry to an HTML log file.
 /*!
@@ -398,7 +398,7 @@ void addLogEntry(std::string tag, std::string value);
  *
  * @ingroup HTML_logs
  */
-void addLogEntry(std::string tag, doublereal value);
+void addLogEntry(const std::string& tag, doublereal value);
 
 //! Add an entry to an HTML log file.
 /*!
@@ -409,7 +409,7 @@ void addLogEntry(std::string tag, doublereal value);
  *
  * @ingroup HTML_logs
  */
-void addLogEntry(std::string tag, int value);
+void addLogEntry(const std::string& tag, int value);
 
 //! Add an entry msg string to an HTML log file.
 /*!
@@ -419,7 +419,7 @@ void addLogEntry(std::string tag, int value);
  *
  * @ingroup HTML_logs
  */
-void addLogEntry(std::string msg);
+void addLogEntry(const std::string& msg);
 
 //! Close the current group of log messages.
 /*!
@@ -434,7 +434,7 @@ void addLogEntry(std::string msg);
  *              log group created.
  * @ingroup HTML_logs
  */
-void endLogGroup(std::string title="");
+void endLogGroup(const std::string& title="");
 
 //! Write the HTML log file.
 /*!
@@ -451,16 +451,16 @@ void endLogGroup(std::string title="");
  *  @param  file Name of the file to be written
  *  @ingroup HTML_logs
  */
-void write_logfile(std::string file = "log.html");
+void write_logfile(const std::string& file = "log.html");
 
 #else
-inline void beginLogGroup(std::string title, int loglevel=-99) {}
-inline void addLogEntry(std::string tag, std::string value) {}
-inline void addLogEntry(std::string tag, doublereal value) {}
-inline void addLogEntry(std::string tag, int value) {}
-inline void addLogEntry(std::string msg) {}
-inline void endLogGroup(std::string title="") {}
-inline void write_logfile(std::string file = "log.html") {}
+inline void beginLogGroup(const std::string& title, int loglevel=-99) {}
+inline void addLogEntry(const std::string& tag, const std::string& value) {}
+inline void addLogEntry(const std::string& tag, doublereal value) {}
+inline void addLogEntry(const std::string& tag, int value) {}
+inline void addLogEntry(const std::string& msg) {}
+inline void endLogGroup(const std::string& title="") {}
+inline void write_logfile(const std::string& file = "log.html") {}
 #endif
 
 //! This routine will locate an XML node in either the input
diff --git a/include/cantera/base/stringUtils.h b/include/cantera/base/stringUtils.h
index 7ccb8d67e..7b37a9a71 100644
--- a/include/cantera/base/stringUtils.h
+++ b/include/cantera/base/stringUtils.h
@@ -162,7 +162,7 @@ std::string getBaseName(const std::string& fullPath);
  *
  *  @return      Returns an integer
  */
-int intValue(std::string val);
+int intValue(const std::string& val);
 
 //! Translate a string into one doublereal value
 /*!
@@ -173,7 +173,7 @@ int intValue(std::string val);
  *
  *  @return      Returns a doublereal value
  */
-doublereal fpValue(std::string val);
+doublereal fpValue(const std::string& val);
 
 //! Translate a string into one doublereal value
 /*!
@@ -183,7 +183,7 @@ doublereal fpValue(std::string val);
  *
  *  @return      Returns a doublereal value
  */
-doublereal fpValueCheck(std::string val);
+doublereal fpValueCheck(const std::string& val);
 
 //! Parse a name string, separating out the phase name from the species name
 /*!
diff --git a/include/cantera/base/xml.h b/include/cantera/base/xml.h
index 32fff945f..6cc6e415f 100644
--- a/include/cantera/base/xml.h
+++ b/include/cantera/base/xml.h
@@ -76,7 +76,7 @@ public:
      *    @param attribs              map of attribute name and
      *                                attribute value - output
      */
-    void parseTag(std::string tag, std::string& name,
+    void parseTag(const std::string& tag, std::string& name,
                   std::map<std::string, std::string>& attribs) const;
 
     //! Reads an XML tag into a string
@@ -302,7 +302,7 @@ public:
      *  @param cname  Name of the child node to the current
      *                node, for which you want the value
      */
-    std::string operator()(std::string cname) const;
+    std::string operator()(const std::string& cname) const;
 
     //! Return the value of an XML node as a single double
     /*!
@@ -427,7 +427,7 @@ public:
      *
      * @return Returns true if the child node exists, false otherwise.
      */
-    bool hasChild(const std::string ch) const;
+    bool hasChild(const std::string& ch) const;
 
     //! Tests whether the current node has an attribute with a particular name
     /*!
@@ -435,7 +435,7 @@ public:
      *
      * @return Returns true if the attribute exists, false otherwise.
      */
-    bool hasAttrib(std::string a) const;
+    bool hasAttrib(const std::string& a) const;
 
     //! Returns the name of the XML node
     /*!
@@ -449,7 +449,7 @@ public:
     /*!
      * @param name The name of the XML node
      */
-    void setName(std::string name) {
+    void setName(const std::string& name) {
         m_name = name;
     }
 
diff --git a/include/cantera/equil/MultiPhase.h b/include/cantera/equil/MultiPhase.h
index f221e013b..489493cd5 100644
--- a/include/cantera/equil/MultiPhase.h
+++ b/include/cantera/equil/MultiPhase.h
@@ -141,7 +141,7 @@ public:
     /*!
      * @param name   String name of the global element
      */
-    size_t elementIndex(std::string name) const;
+    size_t elementIndex(const std::string& name) const;
 
     //! Number of species, summed over all phases.
     size_t nSpecies() const {
@@ -273,7 +273,7 @@ public:
      *  If the species or phase name is not recognized, this routine throws
      *  a CanteraError.
      */
-    size_t speciesIndex(std::string speciesName, std::string phaseName);
+    size_t speciesIndex(const std::string& speciesName, const std::string& phaseName);
 
     /// Minimum temperature for which all solution phases have
     /// valid thermo data. Stoichiometric phases are not
diff --git a/include/cantera/kinetics/Kinetics.h b/include/cantera/kinetics/Kinetics.h
index 2c42c9d5f..032653fb5 100644
--- a/include/cantera/kinetics/Kinetics.h
+++ b/include/cantera/kinetics/Kinetics.h
@@ -256,7 +256,7 @@ public:
      * If a -1 is returned, then the phase is not defined in
      * the Kinetics object.
      */
-    size_t phaseIndex(std::string ph) {
+    size_t phaseIndex(const std::string& ph) {
         if (m_phaseindex.find(ph) == m_phaseindex.end()) {
             return npos;
         } else {
@@ -396,7 +396,7 @@ public:
      *
      * @param nm   String containing the name of the species.
      */
-    thermo_t& speciesPhase(std::string nm);
+    thermo_t& speciesPhase(const std::string& nm);
 
     /**
      * This function takes as an argument the kineticsSpecies index
@@ -1049,7 +1049,7 @@ private:
     /*!
      * @param m String message
      */
-    void err(std::string m) const;
+    void err(const std::string& m) const;
 
 };
 
diff --git a/include/cantera/kinetics/KineticsFactory.h b/include/cantera/kinetics/KineticsFactory.h
index 6ce3a6be7..895641d33 100644
--- a/include/cantera/kinetics/KineticsFactory.h
+++ b/include/cantera/kinetics/KineticsFactory.h
@@ -19,7 +19,7 @@ namespace Cantera
 class UnknownKineticsModel : public CanteraError
 {
 public:
-    UnknownKineticsModel(std::string proc, std::string kineticsModel) :
+    UnknownKineticsModel(const std::string& proc, const std::string& kineticsModel) :
         CanteraError(proc, "Specified Kinetics model "
                      + kineticsModel +
                      " does not match any known type.") {}
@@ -62,7 +62,7 @@ public:
     virtual Kinetics* newKinetics(XML_Node& phase,
                                   std::vector<ThermoPhase*> th);
 
-    virtual Kinetics* newKinetics(std::string model);
+    virtual Kinetics* newKinetics(const std::string& model);
 
 private:
 
@@ -88,7 +88,7 @@ inline Kinetics* newKineticsMgr(XML_Node& phase,
 /**
  *  Create a new kinetics manager.
  */
-inline Kinetics* newKineticsMgr(std::string model, KineticsFactory* f=0)
+inline Kinetics* newKineticsMgr(const std::string& model, KineticsFactory* f=0)
 {
     if (f == 0) {
         f = KineticsFactory::factory();
diff --git a/include/cantera/kinetics/ReactionPath.h b/include/cantera/kinetics/ReactionPath.h
index 01612714f..fbfcb6fd0 100644
--- a/include/cantera/kinetics/ReactionPath.h
+++ b/include/cantera/kinetics/ReactionPath.h
@@ -107,7 +107,8 @@ public:
     /// Destructor
     virtual ~Path() {}
 
-    void addReaction(size_t rxnNumber, doublereal value, std::string label = "");
+    void addReaction(size_t rxnNumber, doublereal value,
+                     const std::string& label = "");
 
     /// Upstream node.
     const SpeciesNode* begin() const {
@@ -213,7 +214,7 @@ public:
         return m_nodes.size();
     }
 
-    void addNode(size_t k, std::string nm, doublereal x = 0.0);
+    void addNode(size_t k, const std::string& nm, doublereal x = 0.0);
 
     void displayOnly(size_t k=npos) {
         m_local = k;
@@ -222,10 +223,10 @@ public:
     void linkNodes(size_t k1, size_t k2, size_t rxn, doublereal value,
                    std::string legend = "");
 
-    void include(std::string aaname) {
+    void include(const std::string& aaname) {
         m_include.push_back(aaname);
     }
-    void exclude(std::string aaname) {
+    void exclude(const std::string& aaname) {
         m_exclude.push_back(aaname);
     }
     void include(std::vector<std::string>& names) {
@@ -247,7 +248,7 @@ public:
     std::vector<size_t> species();
     vector_int reactions();
     void findMajorPaths(doublereal threshold, size_t lda, doublereal* a);
-    void setFont(std::string font) {
+    void setFont(const std::string& font) {
         m_font = font;
     }
     // public attributes
@@ -292,7 +293,7 @@ public:
 
     int init(std::ostream& logfile, Kinetics& s);
 
-    int build(Kinetics& s, std::string element, std::ostream& output,
+    int build(Kinetics& s, const std::string& element, std::ostream& output,
               ReactionPathDiagram& r, bool quiet=false);
 
     int findGroups(std::ostream& logfile, Kinetics& s);
diff --git a/include/cantera/kinetics/ReactionStoichMgr.h b/include/cantera/kinetics/ReactionStoichMgr.h
index 0d6d11c43..fdde35466 100644
--- a/include/cantera/kinetics/ReactionStoichMgr.h
+++ b/include/cantera/kinetics/ReactionStoichMgr.h
@@ -217,7 +217,7 @@ public:
      */
     virtual void multiplyRevProducts(const doublereal* c, doublereal* r);
 
-    virtual void write(std::string filename);
+    virtual void write(const std::string& filename);
 
 protected:
 
diff --git a/include/cantera/kinetics/StoichManager.h b/include/cantera/kinetics/StoichManager.h
index 265044061..145e31c7d 100644
--- a/include/cantera/kinetics/StoichManager.h
+++ b/include/cantera/kinetics/StoichManager.h
@@ -150,7 +150,7 @@ static doublereal ppow(doublereal x, doublereal order)
     }
 }
 
-inline static std::string fmt(std::string r, size_t n)
+inline static std::string fmt(const std::string& r, size_t n)
 {
     return r + "[" + int2str(n) + "]";
 }
@@ -220,21 +220,21 @@ public:
         return 1;
     }
 
-    void writeMultiply(std::string r, std::map<size_t, std::string>& out) {
+    void writeMultiply(const std::string& r, std::map<size_t, std::string>& out) {
         out[m_rxn] = fmt(r, m_ic0);
     }
 
-    void writeIncrementReaction(std::string r, std::map<size_t, std::string>& out) {
+    void writeIncrementReaction(const std::string& r, std::map<size_t, std::string>& out) {
         out[m_rxn] += " + "+fmt(r, m_ic0);
     }
-    void writeDecrementReaction(std::string r, std::map<size_t, std::string>& out) {
+    void writeDecrementReaction(const std::string& r, std::map<size_t, std::string>& out) {
         out[m_rxn] += " - "+fmt(r, m_ic0);
     }
 
-    void writeIncrementSpecies(std::string r, std::map<size_t, std::string>& out) {
+    void writeIncrementSpecies(const std::string& r, std::map<size_t, std::string>& out) {
         out[m_ic0] += " + "+fmt(r, m_rxn);
     }
-    void writeDecrementSpecies(std::string r, std::map<size_t, std::string>& out) {
+    void writeDecrementSpecies(const std::string& r, std::map<size_t, std::string>& out) {
         out[m_ic0] += " - "+fmt(r, m_rxn);
     }
 
@@ -311,22 +311,22 @@ public:
         return 2;
     }
 
-    void writeMultiply(std::string r, std::map<size_t, std::string>& out) {
+    void writeMultiply(const std::string& r, std::map<size_t, std::string>& out) {
         out[m_rxn] = fmt(r, m_ic0) + " * " + fmt(r, m_ic1);
     }
-    void writeIncrementReaction(std::string r, std::map<size_t, std::string>& out) {
+    void writeIncrementReaction(const std::string& r, std::map<size_t, std::string>& out) {
         out[m_rxn] += " + "+fmt(r, m_ic0)+" + "+fmt(r, m_ic1);
     }
-    void writeDecrementReaction(std::string r, std::map<size_t, std::string>& out) {
+    void writeDecrementReaction(const std::string& r, std::map<size_t, std::string>& out) {
         out[m_rxn] += " - "+fmt(r, m_ic0)+" - "+fmt(r, m_ic1);
     }
 
-    void writeIncrementSpecies(std::string r, std::map<size_t, std::string>& out) {
+    void writeIncrementSpecies(const std::string& r, std::map<size_t, std::string>& out) {
         std::string s = " + "+fmt(r, m_rxn);
         out[m_ic0] += s;
         out[m_ic1] += s;
     }
-    void writeDecrementSpecies(std::string r, std::map<size_t, std::string>& out) {
+    void writeDecrementSpecies(const std::string& r, std::map<size_t, std::string>& out) {
         std::string s = " - "+fmt(r, m_rxn);
         out[m_ic0] += s;
         out[m_ic1] += s;
@@ -416,22 +416,22 @@ public:
         return 3;
     }
 
-    void writeMultiply(std::string r, std::map<size_t, std::string>& out) {
+    void writeMultiply(const std::string& r, std::map<size_t, std::string>& out) {
         out[m_rxn] = fmt(r, m_ic0) + " * " + fmt(r, m_ic1) + " * " + fmt(r, m_ic2);
     }
-    void writeIncrementReaction(std::string r, std::map<size_t, std::string>& out) {
+    void writeIncrementReaction(const std::string& r, std::map<size_t, std::string>& out) {
         out[m_rxn] += " + "+fmt(r, m_ic0)+" + "+fmt(r, m_ic1)+" + "+fmt(r, m_ic2);
     }
-    void writeDecrementReaction(std::string r, std::map<size_t, std::string>& out) {
+    void writeDecrementReaction(const std::string& r, std::map<size_t, std::string>& out) {
         out[m_rxn] += " - "+fmt(r, m_ic0)+" - "+fmt(r, m_ic1)+" - "+fmt(r, m_ic2);
     }
-    void writeIncrementSpecies(std::string r, std::map<size_t, std::string>& out) {
+    void writeIncrementSpecies(const std::string& r, std::map<size_t, std::string>& out) {
         std::string s = " + "+fmt(r, m_rxn);
         out[m_ic0] += s;
         out[m_ic1] += s;
         out[m_ic2] += s;
     }
-    void writeDecrementSpecies(std::string r, std::map<size_t, std::string>& out) {
+    void writeDecrementSpecies(const std::string& r, std::map<size_t, std::string>& out) {
         std::string s = " - "+fmt(r, m_rxn);
         out[m_ic0] += s;
         out[m_ic1] += s;
@@ -548,7 +548,7 @@ public:
             -= m_stoich[n]*input[m_ic[n]];
     }
 
-    void writeMultiply(std::string r, std::map<size_t, std::string>& out) {
+    void writeMultiply(const std::string& r, std::map<size_t, std::string>& out) {
         out[m_rxn] = "";
         for (size_t n = 0; n < m_n; n++) {
             if (m_order[n] == 1.0) {
@@ -561,24 +561,24 @@ public:
             }
         }
     }
-    void writeIncrementReaction(std::string r, std::map<size_t, std::string>& out) {
+    void writeIncrementReaction(const std::string& r, std::map<size_t, std::string>& out) {
         for (size_t n = 0; n < m_n; n++) {
             out[m_rxn] += " + "+fp2str(m_stoich[n]) + "*" + fmt(r, m_ic[n]);
         }
     }
-    void writeDecrementReaction(std::string r, std::map<size_t, std::string>& out) {
+    void writeDecrementReaction(const std::string& r, std::map<size_t, std::string>& out) {
         for (size_t n = 0; n < m_n; n++) {
             out[m_rxn] += " - "+fp2str(m_stoich[n]) + "*" + fmt(r, m_ic[n]);
         }
     }
-    void writeIncrementSpecies(std::string r, std::map<size_t, std::string>& out) {
+    void writeIncrementSpecies(const std::string& r, std::map<size_t, std::string>& out) {
         std::string s = fmt(r, m_rxn);
         for (size_t n = 0; n < m_n; n++) {
             out[m_ic[n]] += " + "+fp2str(m_stoich[n]) + "*" + s;
         }
     }
 
-    void writeDecrementSpecies(std::string r, std::map<size_t, std::string>& out) {
+    void writeDecrementSpecies(const std::string& r, std::map<size_t, std::string>& out) {
         std::string s = fmt(r, m_rxn);
         for (size_t n = 0; n < m_n; n++) {
             out[m_ic[n]] += " - "+fp2str(m_stoich[n]) + "*" + s;
@@ -659,8 +659,8 @@ inline static void _decrementReactions(InputIter begin,
 
 
 template<class InputIter>
-inline static void _writeIncrementSpecies(InputIter begin, InputIter end, std::string r,
-        std::map<size_t, std::string>& out)
+inline static void _writeIncrementSpecies(InputIter begin, InputIter end,
+        const std::string& r, std::map<size_t, std::string>& out)
 {
     for (; begin != end; ++begin) {
         begin->writeIncrementSpecies(r, out);
@@ -668,8 +668,8 @@ inline static void _writeIncrementSpecies(InputIter begin, InputIter end, std::s
 }
 
 template<class InputIter>
-inline static void _writeDecrementSpecies(InputIter begin, InputIter end, std::string r,
-        std::map<size_t, std::string>& out)
+inline static void _writeDecrementSpecies(InputIter begin, InputIter end,
+        const std::string& r, std::map<size_t, std::string>& out)
 {
     for (; begin != end; ++begin) {
         begin->writeDecrementSpecies(r, out);
@@ -677,8 +677,8 @@ inline static void _writeDecrementSpecies(InputIter begin, InputIter end, std::s
 }
 
 template<class InputIter>
-inline static void _writeIncrementReaction(InputIter begin, InputIter end, std::string r,
-        std::map<size_t, std::string>& out)
+inline static void _writeIncrementReaction(InputIter begin, InputIter end,
+        const std::string& r, std::map<size_t, std::string>& out)
 {
     for (; begin != end; ++begin) {
         begin->writeIncrementReaction(r, out);
@@ -686,8 +686,8 @@ inline static void _writeIncrementReaction(InputIter begin, InputIter end, std::
 }
 
 template<class InputIter>
-inline static void _writeDecrementReaction(InputIter begin, InputIter end, std::string r,
-        std::map<size_t, std::string>& out)
+inline static void _writeDecrementReaction(InputIter begin, InputIter end,
+        const std::string& r, std::map<size_t, std::string>& out)
 {
     for (; begin != end; ++begin) {
         begin->writeDecrementReaction(r, out);
@@ -695,8 +695,8 @@ inline static void _writeDecrementReaction(InputIter begin, InputIter end, std::
 }
 
 template<class InputIter>
-inline static void _writeMultiply(InputIter begin, InputIter end, std::string r,
-                                  std::map<size_t, std::string>& out)
+inline static void _writeMultiply(InputIter begin, InputIter end,
+        const std::string& r, std::map<size_t, std::string>& out)
 {
     for (; begin != end; ++begin) {
         begin->writeMultiply(r, out);
@@ -891,35 +891,35 @@ public:
         _decrementReactions(m_cn_list.begin(), m_cn_list.end(), input, output);
     }
 
-    void writeIncrementSpecies(std::string r, std::map<size_t, std::string>& out) {
+    void writeIncrementSpecies(const std::string& r, std::map<size_t, std::string>& out) {
         _writeIncrementSpecies(m_c1_list.begin(), m_c1_list.end(), r, out);
         _writeIncrementSpecies(m_c2_list.begin(), m_c2_list.end(), r, out);
         _writeIncrementSpecies(m_c3_list.begin(), m_c3_list.end(), r, out);
         _writeIncrementSpecies(m_cn_list.begin(), m_cn_list.end(), r, out);
     }
 
-    void writeDecrementSpecies(std::string r, std::map<size_t, std::string>& out) {
+    void writeDecrementSpecies(const std::string& r, std::map<size_t, std::string>& out) {
         _writeDecrementSpecies(m_c1_list.begin(), m_c1_list.end(), r, out);
         _writeDecrementSpecies(m_c2_list.begin(), m_c2_list.end(), r, out);
         _writeDecrementSpecies(m_c3_list.begin(), m_c3_list.end(), r, out);
         _writeDecrementSpecies(m_cn_list.begin(), m_cn_list.end(), r, out);
     }
 
-    void writeIncrementReaction(std::string r, std::map<size_t, std::string>& out) {
+    void writeIncrementReaction(const std::string& r, std::map<size_t, std::string>& out) {
         _writeIncrementReaction(m_c1_list.begin(), m_c1_list.end(), r, out);
         _writeIncrementReaction(m_c2_list.begin(), m_c2_list.end(), r, out);
         _writeIncrementReaction(m_c3_list.begin(), m_c3_list.end(), r, out);
         _writeIncrementReaction(m_cn_list.begin(), m_cn_list.end(), r, out);
     }
 
-    void writeDecrementReaction(std::string r, std::map<size_t, std::string>& out) {
+    void writeDecrementReaction(const std::string& r, std::map<size_t, std::string>& out) {
         _writeDecrementReaction(m_c1_list.begin(), m_c1_list.end(), r, out);
         _writeDecrementReaction(m_c2_list.begin(), m_c2_list.end(), r, out);
         _writeDecrementReaction(m_c3_list.begin(), m_c3_list.end(), r, out);
         _writeDecrementReaction(m_cn_list.begin(), m_cn_list.end(), r, out);
     }
 
-    void writeMultiply(std::string r, std::map<size_t, std::string>& out) {
+    void writeMultiply(const std::string& r, std::map<size_t, std::string>& out) {
         _writeMultiply(m_c1_list.begin(), m_c1_list.end(), r, out);
         _writeMultiply(m_c2_list.begin(), m_c2_list.end(), r, out);
         _writeMultiply(m_c3_list.begin(), m_c3_list.end(), r, out);
diff --git a/include/cantera/kinetics/importKinetics.h b/include/cantera/kinetics/importKinetics.h
index aabd2d09d..3f37bdbd7 100644
--- a/include/cantera/kinetics/importKinetics.h
+++ b/include/cantera/kinetics/importKinetics.h
@@ -225,8 +225,8 @@ bool importKinetics(const XML_Node& phase, std::vector<ThermoPhase*> th,
  * @ingroup inputfiles
  * @see importKinetics()
  */
-bool buildSolutionFromXML(XML_Node& root, std::string id, std::string nm,
-                          ThermoPhase* th, Kinetics* k);
+bool buildSolutionFromXML(XML_Node& root, const std::string& id,
+                          const std::string& nm, ThermoPhase* th, Kinetics* k);
 
 //! Search an XML tree for species data.
 /*!
diff --git a/include/cantera/numerics/BEulerInt.h b/include/cantera/numerics/BEulerInt.h
index d57e3f7e0..7bb82eb43 100644
--- a/include/cantera/numerics/BEulerInt.h
+++ b/include/cantera/numerics/BEulerInt.h
@@ -47,7 +47,7 @@ enum BEulerMethodType {
 class BEulerErr : public CanteraError
 {
 public:
-    BEulerErr(std::string msg);
+    explicit BEulerErr(const std::string& msg);
 };
 
 
diff --git a/include/cantera/numerics/BandMatrix.h b/include/cantera/numerics/BandMatrix.h
index f6d1fa3b7..8181a28ac 100644
--- a/include/cantera/numerics/BandMatrix.h
+++ b/include/cantera/numerics/BandMatrix.h
@@ -423,7 +423,7 @@ private:
     /*!
      * @param msg String containing the message.
      */
-    void err(std::string msg) const;
+    void err(const std::string& msg) const;
 
 
 };
diff --git a/include/cantera/numerics/DAE_Solver.h b/include/cantera/numerics/DAE_Solver.h
index 5a8120fed..92f6bc9a4 100644
--- a/include/cantera/numerics/DAE_Solver.h
+++ b/include/cantera/numerics/DAE_Solver.h
@@ -244,7 +244,7 @@ protected:
 
 
 private:
-    void warn(std::string msg) const {
+    void warn(const std::string& msg) const {
         writelog(">>>> Warning: method "+msg+" of base class "
                  +"DAE_Solver called. Nothing done.\n");
     }
@@ -260,7 +260,7 @@ private:
  *
  *     @return       Returns a point to the instantiated DAE_Solver object
  */
-DAE_Solver* newDAE_Solver(std::string itype, ResidJacEval& f);
+DAE_Solver* newDAE_Solver(const std::string& itype, ResidJacEval& f);
 
 #endif
 
diff --git a/include/cantera/numerics/DenseMatrix.h b/include/cantera/numerics/DenseMatrix.h
index 0ddf974dd..ce9030478 100644
--- a/include/cantera/numerics/DenseMatrix.h
+++ b/include/cantera/numerics/DenseMatrix.h
@@ -40,7 +40,7 @@ public:
      *  @param routine  Name of calling routine
      *  @param msg      Informative message
      */
-    CELapackError(std::string routine, std::string msg) :
+    CELapackError(const std::string& routine, const std::string& msg) :
         CanteraError(routine + " LAPACK ERROR", msg) {
     }
 
diff --git a/include/cantera/numerics/Func1.h b/include/cantera/numerics/Func1.h
index e2b2e68c3..595d6b0ae 100644
--- a/include/cantera/numerics/Func1.h
+++ b/include/cantera/numerics/Func1.h
@@ -89,7 +89,7 @@ public:
     virtual doublereal isProportional(TimesConstant1& other);
     virtual doublereal isProportional(Func1& other);
 
-    virtual std::string write(std::string arg) const;
+    virtual std::string write(const std::string& arg) const;
 
 
     //! accessor function for the stored constant
@@ -166,7 +166,7 @@ public:
         return (Func1&) *nfunc;
     }
 
-    virtual std::string write(std::string arg) const;
+    virtual std::string write(const std::string& arg) const;
 
     virtual int ID() const {
         return SinFuncType;
@@ -204,7 +204,7 @@ public:
         Cos1* nfunc = new Cos1(*this);
         return (Func1&) *nfunc;
     }
-    virtual std::string write(std::string arg) const;
+    virtual std::string write(const std::string& arg) const;
     virtual int ID() const {
         return CosFuncType;
     }
@@ -236,7 +236,7 @@ public:
         Func1::operator=(right);
         return *this;
     }
-    virtual std::string write(std::string arg) const;
+    virtual std::string write(const std::string& arg) const;
     virtual int ID() const {
         return ExpFuncType;
     }
@@ -272,7 +272,7 @@ public:
         Func1::operator=(right);
         return *this;
     }
-    virtual std::string write(std::string arg) const;
+    virtual std::string write(const std::string& arg) const;
     virtual int ID() const {
         return PowFuncType;
     }
@@ -313,7 +313,7 @@ public:
         return *this;
     }
 
-    virtual std::string write(std::string arg) const;
+    virtual std::string write(const std::string& arg) const;
     virtual int ID() const {
         return ConstFuncType;
     }
@@ -394,7 +394,7 @@ public:
         return 0;
     }
 
-    virtual std::string write(std::string arg) const;
+    virtual std::string write(const std::string& arg) const;
 };
 
 
@@ -456,7 +456,7 @@ public:
         return 0;
     }
 
-    virtual std::string write(std::string arg) const;
+    virtual std::string write(const std::string& arg) const;
 
 };
 
@@ -509,7 +509,7 @@ public:
         return dup;
     }
 
-    virtual std::string write(std::string arg) const;
+    virtual std::string write(const std::string& arg) const;
 
     virtual doublereal eval(doublereal t) const {
         return m_f1->eval(t) * m_f2->eval(t);
@@ -596,7 +596,7 @@ public:
         return *d;
     }
 
-    virtual std::string write(std::string arg) const;
+    virtual std::string write(const std::string& arg) const;
 
     virtual int order() const {
         return 0;
@@ -654,7 +654,7 @@ public:
         Func1& f1d = m_f1->derivative();
         return f1d;
     }
-    virtual std::string write(std::string arg) const;
+    virtual std::string write(const std::string& arg) const;
 
     virtual int order() const {
         return 0;
@@ -725,7 +725,7 @@ public:
         return r;
     }
 
-    virtual std::string write(std::string arg) const;
+    virtual std::string write(const std::string& arg) const;
 
     virtual int order() const {
         return 1;
@@ -795,7 +795,7 @@ public:
         return *p;
     }
 
-    virtual std::string write(std::string arg) const;
+    virtual std::string write(const std::string& arg) const;
 
     virtual int order() const {
         return 2;
diff --git a/include/cantera/numerics/IDA_Solver.h b/include/cantera/numerics/IDA_Solver.h
index b594f8e6f..4b4cf5b3e 100644
--- a/include/cantera/numerics/IDA_Solver.h
+++ b/include/cantera/numerics/IDA_Solver.h
@@ -40,7 +40,7 @@ namespace Cantera
 class IDA_Err : public CanteraError
 {
 public:
-    IDA_Err(std::string msg) : CanteraError("IDA_Solver", msg) {}
+    explicit IDA_Err(const std::string& msg) : CanteraError("IDA_Solver", msg) {}
 };
 
 
diff --git a/include/cantera/numerics/Integrator.h b/include/cantera/numerics/Integrator.h
index 301c88f26..2ecbabb3d 100644
--- a/include/cantera/numerics/Integrator.h
+++ b/include/cantera/numerics/Integrator.h
@@ -205,7 +205,7 @@ public:
 private:
 
     doublereal m_dummy;
-    void warn(std::string msg) const {
+    void warn(const std::string& msg) const {
         writelog(">>>> Warning: method "+msg+" of base class "
                  +"Integrator called. Nothing done.\n");
     }
@@ -213,7 +213,7 @@ private:
 };
 
 // defined in ODE_integrators.cpp
-Integrator* newIntegrator(std::string itype);
+Integrator* newIntegrator(const std::string& itype);
 
 // defined in ODE_integrators.cpp
 void deleteIntegrator(Integrator* cv);
diff --git a/include/cantera/oneD/Domain1D.h b/include/cantera/oneD/Domain1D.h
index 1e11db3ed..ddcc63b06 100644
--- a/include/cantera/oneD/Domain1D.h
+++ b/include/cantera/oneD/Domain1D.h
@@ -225,7 +225,7 @@ public:
         }
     }
 
-    void setComponentName(size_t n, std::string name) {
+    void setComponentName(size_t n, const std::string& name) {
         m_name[n] = name;
     }
 
@@ -236,7 +236,7 @@ public:
     }
 
     /// index of component with name \a name.
-    size_t componentIndex(std::string name) const {
+    size_t componentIndex(const std::string& name) const {
         size_t nc = nComponents();
         for (size_t n = 0; n < nc; n++) {
             if (name == componentName(n)) {
@@ -543,7 +543,7 @@ public:
     }
 
 
-    void setProfile(std::string name, doublereal* values, doublereal* soln) {
+    void setProfile(const std::string& name, doublereal* values, doublereal* soln) {
         for (size_t n = 0; n < m_nv; n++) {
             if (name == componentName(n)) {
                 for (size_t j = 0; j < m_points; j++) {
diff --git a/include/cantera/oneD/Inlet1D.h b/include/cantera/oneD/Inlet1D.h
index 190c1227a..3a77777eb 100644
--- a/include/cantera/oneD/Inlet1D.h
+++ b/include/cantera/oneD/Inlet1D.h
@@ -59,7 +59,7 @@ public:
     }
 
     /// Set the mole fractions by specifying a std::string.
-    virtual void setMoleFractions(std::string xin) {
+    virtual void setMoleFractions(const std::string& xin) {
         err("setMoleFractions");
     }
 
@@ -105,7 +105,7 @@ protected:
     doublereal m_temp, m_mdot;
 
 private:
-    void err(std::string method) {
+    void err(const std::string& method) {
         throw CanteraError("Bdry1D::"+method,
                            "attempt to call base class method "+method);
     }
@@ -169,7 +169,7 @@ public:
 
     virtual void _finalize(const doublereal* x) {}
 
-    virtual void setMoleFractions(std::string xin);
+    virtual void setMoleFractions(const std::string& xin);
     virtual void setMoleFractions(doublereal* xin);
     virtual doublereal massFraction(size_t k) {
         return m_yin[k];
@@ -319,7 +319,7 @@ public:
         ;
     }
 
-    virtual void setMoleFractions(std::string xin);
+    virtual void setMoleFractions(const std::string& xin);
     virtual void setMoleFractions(doublereal* xin);
     virtual doublereal massFraction(size_t k) {
         return m_yres[k];
diff --git a/include/cantera/oneD/OneDim.h b/include/cantera/oneD/OneDim.h
index 560a60d49..e8471e72c 100644
--- a/include/cantera/oneD/OneDim.h
+++ b/include/cantera/oneD/OneDim.h
@@ -58,7 +58,7 @@ public:
         return *m_dom[i];
     }
 
-    size_t domainIndex(std::string name);
+    size_t domainIndex(const std::string& name);
 
     //! Check that the specified domain index is in range
     //! Throws an exception if n is greater than nDomains()-1
@@ -192,7 +192,8 @@ public:
      */
     void writeStats(int printTime = 1);
 
-    void save(std::string fname, std::string id, std::string desc, doublereal* sol);
+    void save(const std::string& fname, std::string id,
+              const std::string& desc, doublereal* sol);
 
     // options
     void setMinTimeStep(doublereal tmin) {
diff --git a/include/cantera/oneD/Sim1D.h b/include/cantera/oneD/Sim1D.h
index 083f189c2..5276a04ff 100644
--- a/include/cantera/oneD/Sim1D.h
+++ b/include/cantera/oneD/Sim1D.h
@@ -52,7 +52,8 @@ public:
     //@{
 
     /// Set initial guess based on equilibrium
-    void setInitialGuess(std::string component, vector_fp& locs, vector_fp& vals);
+    void setInitialGuess(const std::string& component, vector_fp& locs,
+                         vector_fp& vals);
 
     /// Set one entry in the solution vector.
     void setValue(size_t dom, size_t comp, size_t localPoint,  doublereal value);
@@ -71,7 +72,8 @@ public:
 
     //@}
 
-    void save(std::string fname, std::string id, std::string desc);
+    void save(const std::string& fname, const std::string& id,
+              const std::string& desc);
 
     /// Print to stream s the current solution for all domains.
     void showSolution(std::ostream& s);
@@ -102,7 +104,7 @@ public:
                            doublereal slope = 0.8, doublereal curve = 0.8, doublereal prune = -0.1);
     void setMaxGridPoints(int dom = -1, int npoints = 300);
 
-    void restore(std::string fname, std::string id);
+    void restore(const std::string& fname, const std::string& id);
     void getInitialSoln();
 
     void setSolution(const doublereal* soln) {
diff --git a/include/cantera/oneD/StFlow.h b/include/cantera/oneD/StFlow.h
index f0a1e9bf5..6cdf5498c 100644
--- a/include/cantera/oneD/StFlow.h
+++ b/include/cantera/oneD/StFlow.h
@@ -177,7 +177,7 @@ public:
 
     virtual std::string componentName(size_t n) const;
 
-    size_t componentIndex(std::string name) const;
+    size_t componentIndex(const std::string& name) const;
 
 
     virtual void showSolution(const doublereal* x);
diff --git a/include/cantera/thermo/AdsorbateThermo.h b/include/cantera/thermo/AdsorbateThermo.h
index ea3fc0704..bb1cf28f5 100644
--- a/include/cantera/thermo/AdsorbateThermo.h
+++ b/include/cantera/thermo/AdsorbateThermo.h
@@ -81,7 +81,7 @@ public:
         return (SpeciesThermoInterpType*) np;
     }
 
-    virtual void install(std::string name, size_t index, int type,
+    virtual void install(const std::string& name, size_t index, int type,
                          const doublereal* c, doublereal minTemp, doublereal maxTemp,
                          doublereal refPressure) {
         m_be = c[1];
diff --git a/include/cantera/thermo/DebyeHuckel.h b/include/cantera/thermo/DebyeHuckel.h
index ca747c182..06c46a57b 100644
--- a/include/cantera/thermo/DebyeHuckel.h
+++ b/include/cantera/thermo/DebyeHuckel.h
@@ -625,7 +625,7 @@ public:
      *  @param id     id attribute containing the name of the phase.
      *                (default is the empty string)
      */
-    DebyeHuckel(std::string inputFile, std::string id = "");
+    DebyeHuckel(const std::string& inputFile, const std::string& id = "");
 
     //! Full constructor for creating the phase.
     /*!
@@ -633,7 +633,7 @@ public:
      *  @param id     id attribute containing the name of the phase.
      *                (default is the empty string)
      */
-    DebyeHuckel(XML_Node& phaseRef, std::string id = "");
+    DebyeHuckel(XML_Node& phaseRef, const std::string& id = "");
 
     /// Destructor.
     virtual ~DebyeHuckel();
@@ -1279,7 +1279,7 @@ public:
      *             to see if phaseNode is pointing to the phase
      *             with the correct id.
      */
-    virtual void  initThermoXML(XML_Node& phaseNode, std::string id);
+    virtual void  initThermoXML(XML_Node& phaseNode, const std::string& id);
 
     //! Return  the Debye Huckel constant as a function of temperature
     //! and pressure (Units = sqrt(kg/gmol))
@@ -1670,7 +1670,7 @@ protected:
     mutable vector_fp m_dlnActCoeffMolaldP;
 
 private:
-    doublereal err(std::string msg) const;
+    doublereal err(const std::string& msg) const;
 
     //! Initialize the internal lengths.
     /*!
diff --git a/include/cantera/thermo/Elements.h b/include/cantera/thermo/Elements.h
index 9e94f016d..f57b275c2 100644
--- a/include/cantera/thermo/Elements.h
+++ b/include/cantera/thermo/Elements.h
@@ -245,7 +245,7 @@ public:
      *
      * @param name String containing the index.
      */
-    int elementIndex(std::string name) const;
+    int elementIndex(const std::string& name) const;
 
     //! Name of the element with index \c m.
     /*!
diff --git a/include/cantera/thermo/FixedChemPotSSTP.h b/include/cantera/thermo/FixedChemPotSSTP.h
index f73e61ebb..18f0327d1 100644
--- a/include/cantera/thermo/FixedChemPotSSTP.h
+++ b/include/cantera/thermo/FixedChemPotSSTP.h
@@ -172,7 +172,7 @@ public:
      * @param id     name of the phase id in the file.
      *               If this is blank, the first phase in the file is used.
      */
-    FixedChemPotSSTP(std::string infile, std::string id = "");
+    FixedChemPotSSTP(const std::string& infile, std::string id = "");
 
     //! Construct and initialize a FixedChemPotSSTP ThermoPhase object
     //! directly from an XML database
@@ -180,7 +180,7 @@ public:
      *  @param phaseRef XML node pointing to a FixedChemPotSSTP description
      *  @param id       Id of the phase.
      */
-    FixedChemPotSSTP(XML_Node& phaseRef, std::string id = "");
+    FixedChemPotSSTP(XML_Node& phaseRef, const std::string& id = "");
 
     //! Copy constructor
     /*!
@@ -198,7 +198,7 @@ public:
      *  @param Ename String name of the element
      *  @param chemPot  Value of the chemical potential of that element (J/kmol)
      */
-    FixedChemPotSSTP(std::string Ename, doublereal chemPot);
+    FixedChemPotSSTP(const std::string& Ename, doublereal chemPot);
 
     //! Assignment operator
     /*!
@@ -575,7 +575,7 @@ public:
     virtual void initThermo();
 
 
-    virtual void initThermoXML(XML_Node& phaseNode, std::string id);
+    virtual void initThermoXML(XML_Node& phaseNode, const std::string& id);
 
     //! Set the equation of state parameters
     /*!
diff --git a/include/cantera/thermo/GeneralSpeciesThermo.h b/include/cantera/thermo/GeneralSpeciesThermo.h
index 6234e3514..b359c922b 100644
--- a/include/cantera/thermo/GeneralSpeciesThermo.h
+++ b/include/cantera/thermo/GeneralSpeciesThermo.h
@@ -85,7 +85,7 @@ public:
      * @todo Create a factory method for SpeciesThermoInterpType.
      *       That's basically what we are doing here.
      */
-    virtual void install(std::string name, size_t index, int type,
+    virtual void install(const std::string& name, size_t index, int type,
                          const doublereal* c,
                          doublereal minTemp, doublereal maxTemp,
                          doublereal refPressure);
diff --git a/include/cantera/thermo/GibbsExcessVPSSTP.h b/include/cantera/thermo/GibbsExcessVPSSTP.h
index 29927a773..b331172e3 100644
--- a/include/cantera/thermo/GibbsExcessVPSSTP.h
+++ b/include/cantera/thermo/GibbsExcessVPSSTP.h
@@ -576,7 +576,7 @@ private:
      *
      * @param msg  Message to be printed
      */
-    doublereal err(std::string msg) const;
+    doublereal err(const std::string& msg) const;
 
 protected:
 
diff --git a/include/cantera/thermo/HMWSoln.h b/include/cantera/thermo/HMWSoln.h
index 24e1632a7..d0783fe9d 100644
--- a/include/cantera/thermo/HMWSoln.h
+++ b/include/cantera/thermo/HMWSoln.h
@@ -1254,7 +1254,7 @@ public:
      * @param id        ID of the phase in the input file. Defaults to the
      *                  empty string.
      */
-    HMWSoln(std::string inputFile, std::string id = "");
+    HMWSoln(const std::string& inputFile, const std::string& id = "");
 
     //! Construct and initialize an HMWSoln ThermoPhase object
     //! directly from an XML database
@@ -1263,7 +1263,7 @@ public:
      *  @param id     id attribute containing the name of the phase.
      *                (default is the empty string)
      */
-    HMWSoln(XML_Node& phaseRef, std::string id = "");
+    HMWSoln(XML_Node& phaseRef, const std::string& id = "");
 
 
     //! Copy Constructor
@@ -2078,7 +2078,7 @@ public:
      *             to see if phaseNode is pointing to the phase
      *             with the correct id.
      */
-    virtual void initThermoXML(XML_Node& phaseNode, std::string id);
+    virtual void initThermoXML(XML_Node& phaseNode, const std::string& id);
 
     //! Value of the Debye Huckel constant as a function of temperature
     //! and pressure.
@@ -3249,7 +3249,7 @@ private:
     /*!
      * @param msg print out a message and error exit
      */
-    doublereal err(std::string msg) const;
+    doublereal err(const std::string& msg) const;
 
     //!  Initialize all of the species - dependent lengths in the object
     void initLengths();
@@ -3512,7 +3512,7 @@ private:
     /*!
      *  @param estString string name of the electrolyte species type
      */
-    static int interp_est(std::string estString);
+    static int interp_est(const std::string& estString);
 
 public:
     /*!
diff --git a/include/cantera/thermo/IdealMolalSoln.h b/include/cantera/thermo/IdealMolalSoln.h
index 6e3d781e4..ab7ea08b5 100644
--- a/include/cantera/thermo/IdealMolalSoln.h
+++ b/include/cantera/thermo/IdealMolalSoln.h
@@ -127,7 +127,7 @@ public:
      *  @param inputFile   Name of the Input file that contains information about the phase
      *  @param id          id of the phase within the input file
      */
-    IdealMolalSoln(std::string inputFile, std::string id = "");
+    IdealMolalSoln(const std::string& inputFile, const std::string& id = "");
 
     //! Constructor for phase initialization
     /*!
@@ -138,7 +138,7 @@ public:
      *                     the information necessary to initialize the phase.
      *  @param id          id of the phase within the input file
      */
-    IdealMolalSoln(XML_Node& phaseRef, std::string id = "");
+    IdealMolalSoln(XML_Node& phaseRef, const std::string& id = "");
 
     /// Destructor.
     virtual ~IdealMolalSoln();
@@ -807,7 +807,7 @@ public:
      *             to see if phaseNode is pointing to the phase
      *             with the correct id.
      */
-    virtual void initThermoXML(XML_Node& phaseNode, std::string id="");
+    virtual void initThermoXML(XML_Node& phaseNode, const std::string& id="");
 
     //! Report the molar volume of species k
     /*!
@@ -926,7 +926,7 @@ private:
     /*!
      *  @param          msg message to be printed
      */
-    doublereal err(std::string msg) const;
+    doublereal err(const std::string& msg) const;
 
     //! This function will be called to update the internally stored
     //! natural logarithm of the molality activity coefficients
diff --git a/include/cantera/thermo/IdealSolidSolnPhase.h b/include/cantera/thermo/IdealSolidSolnPhase.h
index 8d73b84ed..9ee18cf18 100644
--- a/include/cantera/thermo/IdealSolidSolnPhase.h
+++ b/include/cantera/thermo/IdealSolidSolnPhase.h
@@ -107,7 +107,7 @@ public:
      * @param formCG This parameter initializes the m_formGC variable. The default
      *               is a value of 0.
      */
-    IdealSolidSolnPhase(std::string infile, std::string id="", int formCG=0);
+    IdealSolidSolnPhase(const std::string& infile, const std::string& id="", int formCG=0);
 
     //! Construct and initialize an IdealSolidSolnPhase ThermoPhase object
     //! directly from an XML database
@@ -131,7 +131,7 @@ public:
      * @param formCG This parameter initializes the m_formGC variable. The default
      *               is a value of 0.
      */
-    IdealSolidSolnPhase(XML_Node& root, std::string id="", int formCG=0);
+    IdealSolidSolnPhase(XML_Node& root, const std::string& id="", int formCG=0);
 
     /*!
      * Copy Constructor
@@ -921,7 +921,7 @@ public:
       *             to see if phaseNode is pointing to the phase
       *             with the correct id.
       */
-    virtual void initThermoXML(XML_Node& phaseNode, std::string id);
+    virtual void initThermoXML(XML_Node& phaseNode, const std::string& id);
 
 
     /**
diff --git a/include/cantera/thermo/IdealSolnGasVPSS.h b/include/cantera/thermo/IdealSolnGasVPSS.h
index 62b57c631..40fd06e9e 100644
--- a/include/cantera/thermo/IdealSolnGasVPSS.h
+++ b/include/cantera/thermo/IdealSolnGasVPSS.h
@@ -55,7 +55,7 @@ public:
     /// Constructor.
     IdealSolnGasVPSS();
 
-    IdealSolnGasVPSS(std::string infile, std::string id="");
+    IdealSolnGasVPSS(const std::string& infile, std::string id="");
 
     /// Copy Constructor.
     IdealSolnGasVPSS(const IdealSolnGasVPSS&);
@@ -410,7 +410,7 @@ public:
      *             to see if phaseNode is pointing to the phase
      *             with the correct id.
      */
-    virtual void initThermoXML(XML_Node& phaseNode, std::string id);
+    virtual void initThermoXML(XML_Node& phaseNode, const std::string& id);
 
 private:
     //!  @internal Initialize the internal lengths in this object.
diff --git a/include/cantera/thermo/IonsFromNeutralVPSSTP.h b/include/cantera/thermo/IonsFromNeutralVPSSTP.h
index f85b04e2c..348652724 100644
--- a/include/cantera/thermo/IonsFromNeutralVPSSTP.h
+++ b/include/cantera/thermo/IonsFromNeutralVPSSTP.h
@@ -107,7 +107,8 @@ public:
      *                       If this parameter is zero, then a slave
      *                       neutral phase object is created and used.
      */
-    IonsFromNeutralVPSSTP(std::string inputFile, std::string id = "",
+    IonsFromNeutralVPSSTP(const std::string& inputFile,
+                          const std::string& id = "",
                           ThermoPhase* neutralPhase = 0);
 
     //! Construct and initialize an IonsFromNeutralVPSSTP object
@@ -127,7 +128,7 @@ public:
      *                       If this parameter is zero, then a slave
      *                       neutral phase object is created and used.
      */
-    IonsFromNeutralVPSSTP(XML_Node& phaseRoot,  std::string id = "",
+    IonsFromNeutralVPSSTP(XML_Node& phaseRoot, const std::string& id = "",
                           ThermoPhase* neutralPhase = 0);
 
 
@@ -611,7 +612,7 @@ public:
      *             to see if phaseNode is pointing to the phase
      *             with the correct id.
      */
-    void initThermoXML(XML_Node& phaseNode, std::string id);
+    void initThermoXML(XML_Node& phaseNode, const std::string& id);
 
 
 private:
@@ -678,7 +679,7 @@ private:
      *
      * @param msg  Message to be printed
      */
-    doublereal err(std::string msg) const;
+    doublereal err(const std::string& msg) const;
 
 protected:
 
diff --git a/include/cantera/thermo/LatticePhase.h b/include/cantera/thermo/LatticePhase.h
index f2e7f206f..7da310b31 100644
--- a/include/cantera/thermo/LatticePhase.h
+++ b/include/cantera/thermo/LatticePhase.h
@@ -272,14 +272,14 @@ public:
      * @param inputFile String name of the input file
      * @param id        string id of the phase name
      */
-    LatticePhase(std::string inputFile, std::string id = "");
+    LatticePhase(const std::string& inputFile, const std::string& id = "");
 
     //! Full constructor for a water phase
     /*!
      * @param phaseRef  XML node referencing the lattice phase.
      * @param id        string id of the phase name
      */
-    LatticePhase(XML_Node& phaseRef, std::string id = "");
+    LatticePhase(XML_Node& phaseRef, const std::string& id = "");
 
     //! Destructor
     virtual ~LatticePhase();
@@ -869,7 +869,7 @@ public:
      *             to see if phaseNode is pointing to the phase
      *             with the correct id.
      */
-    virtual void initThermoXML(XML_Node& phaseNode, std::string id);
+    virtual void initThermoXML(XML_Node& phaseNode, const std::string& id);
 
     //! Set the equation of state parameters from the argument list
     /*!
diff --git a/include/cantera/thermo/LatticeSolidPhase.h b/include/cantera/thermo/LatticeSolidPhase.h
index d8e4bc637..6d19159db 100644
--- a/include/cantera/thermo/LatticeSolidPhase.h
+++ b/include/cantera/thermo/LatticeSolidPhase.h
@@ -641,7 +641,7 @@ public:
      *  @param x     string containing Name:value pairs that will specify the mole fractions
      *               of species on a particular lattice
      */
-    void setLatticeMoleFractionsByName(int n, std::string x);
+    void setLatticeMoleFractionsByName(int n, const std::string& x);
 
 #ifdef H298MODIFY_CAPABILITY
 
@@ -663,7 +663,7 @@ private:
      *
      *  @return nothing
      */
-    doublereal err(std::string msg) const;
+    doublereal err(const std::string& msg) const;
 
 protected:
 
diff --git a/include/cantera/thermo/MargulesVPSSTP.h b/include/cantera/thermo/MargulesVPSSTP.h
index 24b5038d4..6e356114f 100644
--- a/include/cantera/thermo/MargulesVPSSTP.h
+++ b/include/cantera/thermo/MargulesVPSSTP.h
@@ -339,7 +339,7 @@ public:
      * @param id        ID of the phase in the input file. Defaults to the
      *                  empty string.
      */
-    MargulesVPSSTP(std::string inputFile, std::string id = "");
+    MargulesVPSSTP(const std::string& inputFile, const std::string& id = "");
 
     //! Construct and initialize a MargulesVPSSTP ThermoPhase object
     //! directly from an XML database
@@ -348,7 +348,7 @@ public:
      *  @param id     id attribute containing the name of the phase.
      *                (default is the empty string)
      */
-    MargulesVPSSTP(XML_Node& phaseRef, std::string id = "");
+    MargulesVPSSTP(XML_Node& phaseRef, const std::string& id = "");
 
 
     //! Special constructor for a hard-coded problem
@@ -686,7 +686,7 @@ public:
      *             to see if phaseNode is pointing to the phase
      *             with the correct id.
      */
-    void initThermoXML(XML_Node& phaseNode, std::string id);
+    void initThermoXML(XML_Node& phaseNode, const std::string& id);
 
     /**
      * @}
@@ -839,7 +839,7 @@ private:
      *
      * @param msg  Message to be printed
      */
-    doublereal err(std::string msg) const;
+    doublereal err(const std::string& msg) const;
 
 protected:
 
diff --git a/include/cantera/thermo/MetalSHEelectrons.h b/include/cantera/thermo/MetalSHEelectrons.h
index 8e2525e3c..9c17abe4b 100644
--- a/include/cantera/thermo/MetalSHEelectrons.h
+++ b/include/cantera/thermo/MetalSHEelectrons.h
@@ -198,7 +198,7 @@ public:
      * @param id     name of the phase id in the file.
      *               If this is blank, the first phase in the file is used.
      */
-    MetalSHEelectrons(std::string infile, std::string id = "");
+    MetalSHEelectrons(const std::string& infile, std::string id = "");
 
     //! Construct and initialize a MetalSHEelectrons ThermoPhase object
     //! directly from an XML database
@@ -206,7 +206,7 @@ public:
      *  @param phaseRef XML node pointing to a MetalSHEelectrons description
      *  @param id       Id of the phase.
      */
-    MetalSHEelectrons(XML_Node& phaseRef, std::string id = "");
+    MetalSHEelectrons(XML_Node& phaseRef, const std::string& id = "");
 
     //! Copy constructor
     /*!
@@ -491,7 +491,7 @@ public:
     virtual void initThermo();
 
 
-    virtual void initThermoXML(XML_Node& phaseNode, std::string id);
+    virtual void initThermoXML(XML_Node& phaseNode, const std::string& id);
 
     //! Make the default XML tree
     /*!
diff --git a/include/cantera/thermo/MineralEQ3.h b/include/cantera/thermo/MineralEQ3.h
index 8fef1116b..f07d0f26d 100644
--- a/include/cantera/thermo/MineralEQ3.h
+++ b/include/cantera/thermo/MineralEQ3.h
@@ -174,7 +174,7 @@ public:
      * @param id     name of the phase id in the file.
      *               If this is blank, the first phase in the file is used.
      */
-    MineralEQ3(std::string infile, std::string id = "");
+    MineralEQ3(const std::string& infile, std::string id = "");
 
     //! Construct and initialize a StoichSubstanceSSTP ThermoPhase object
     //! directly from an XML database
@@ -182,7 +182,7 @@ public:
      *  @param phaseRef XML node pointing to a StoichSubstanceSSTP description
      *  @param id       Id of the phase.
      */
-    MineralEQ3(XML_Node& phaseRef, std::string id = "");
+    MineralEQ3(XML_Node& phaseRef, const std::string& id = "");
 
     //! Copy constructor
     /*!
@@ -487,7 +487,7 @@ public:
      *             to see if phaseNode is pointing to the phase
      *             with the correct id.
      */
-    virtual void initThermoXML(XML_Node& phaseNode, std::string id);
+    virtual void initThermoXML(XML_Node& phaseNode, const std::string& id);
 
     //! Set the equation of state parameters
     /*!
diff --git a/include/cantera/thermo/MixedSolventElectrolyte.h b/include/cantera/thermo/MixedSolventElectrolyte.h
index 7ba727425..a8076592c 100644
--- a/include/cantera/thermo/MixedSolventElectrolyte.h
+++ b/include/cantera/thermo/MixedSolventElectrolyte.h
@@ -340,7 +340,8 @@ public:
      * @param id        ID of the phase in the input file. Defaults to the
      *                  empty string.
      */
-    MixedSolventElectrolyte(std::string inputFile, std::string id = "");
+    MixedSolventElectrolyte(const std::string& inputFile,
+                            const std::string& id = "");
 
     //! Construct and initialize a MixedSolventElectrolyte ThermoPhase object
     //! directly from an XML database
@@ -349,7 +350,7 @@ public:
      *  @param id     id attribute containing the name of the phase.
      *                (default is the empty string)
      */
-    MixedSolventElectrolyte(XML_Node& phaseRef, std::string id = "");
+    MixedSolventElectrolyte(XML_Node& phaseRef, const std::string& id = "");
 
 
     //! Special constructor for a hard-coded problem
@@ -688,7 +689,7 @@ public:
      *             to see if phaseNode is pointing to the phase
      *             with the correct id.
      */
-    void initThermoXML(XML_Node& phaseNode, std::string id);
+    void initThermoXML(XML_Node& phaseNode, const std::string& id);
 
     /**
      * @}
@@ -841,7 +842,7 @@ private:
      *
      * @param msg  Message to be printed
      */
-    doublereal err(std::string msg) const;
+    doublereal err(const std::string& msg) const;
 
 protected:
 
diff --git a/include/cantera/thermo/MixtureFugacityTP.h b/include/cantera/thermo/MixtureFugacityTP.h
index 17b078c1f..f838de033 100644
--- a/include/cantera/thermo/MixtureFugacityTP.h
+++ b/include/cantera/thermo/MixtureFugacityTP.h
@@ -688,7 +688,7 @@ public:
      *             to see if phaseNode is pointing to the phase
      *             with the correct id.
      */
-    virtual void initThermoXML(XML_Node& phaseNode, std::string id);
+    virtual void initThermoXML(XML_Node& phaseNode, const std::string& id);
 
 
 private:
@@ -947,7 +947,7 @@ private:
     /*!
      * @param msg  Error message string
      */
-    doublereal err(std::string msg) const;
+    doublereal err(const std::string& msg) const;
 
 };
 }
diff --git a/include/cantera/thermo/MolalityVPSSTP.h b/include/cantera/thermo/MolalityVPSSTP.h
index 58f7e7b4e..4e5a75b7e 100644
--- a/include/cantera/thermo/MolalityVPSSTP.h
+++ b/include/cantera/thermo/MolalityVPSSTP.h
@@ -760,7 +760,7 @@ public:
      *             to see if phaseNode is pointing to the phase
      *             with the correct id.
      */
-    void initThermoXML(XML_Node& phaseNode, std::string id);
+    void initThermoXML(XML_Node& phaseNode, const std::string& id);
 
 
     //! Set the temperature (K), pressure (Pa), and molalities
@@ -931,7 +931,7 @@ private:
      *
      * @param msg  Message to be printed
      */
-    doublereal err(std::string msg) const;
+    doublereal err(const std::string& msg) const;
 
 };
 
diff --git a/include/cantera/thermo/MolarityIonicVPSSTP.h b/include/cantera/thermo/MolarityIonicVPSSTP.h
index e94897e3e..c268d390b 100644
--- a/include/cantera/thermo/MolarityIonicVPSSTP.h
+++ b/include/cantera/thermo/MolarityIonicVPSSTP.h
@@ -85,7 +85,7 @@ public:
      * @param id        ID of the phase in the input file. Defaults to the
      *                  empty string.
      */
-    MolarityIonicVPSSTP(std::string inputFile, std::string id = "");
+    MolarityIonicVPSSTP(const std::string& inputFile, const std::string& id = "");
 
     //! Construct and initialize a MolarityIonicVPSSTP ThermoPhase object
     //! directly from an XML database
@@ -94,7 +94,7 @@ public:
      *  @param id     id attribute containing the name of the phase.
      *                (default is the empty string)
      */
-    MolarityIonicVPSSTP(XML_Node& phaseRef, std::string id = "");
+    MolarityIonicVPSSTP(XML_Node& phaseRef, const std::string& id = "");
 
 
     //! Copy constructor
@@ -385,7 +385,7 @@ public:
      *             to see if phaseNode is pointing to the phase
      *             with the correct id.
      */
-    void initThermoXML(XML_Node& phaseNode, std::string id);
+    void initThermoXML(XML_Node& phaseNode, const std::string& id);
 
 
     //! returns a summary of the state of the phase as a string
@@ -449,7 +449,7 @@ private:
      *
      * @param msg  Message to be printed
      */
-    doublereal err(std::string msg) const;
+    doublereal err(const std::string& msg) const;
 
 protected:
 
diff --git a/include/cantera/thermo/Mu0Poly.h b/include/cantera/thermo/Mu0Poly.h
index c25a835d4..7955b6f80 100644
--- a/include/cantera/thermo/Mu0Poly.h
+++ b/include/cantera/thermo/Mu0Poly.h
@@ -297,7 +297,7 @@ private:
  *
  *  @ingroup spthermo
  */
-void installMu0ThermoFromXML(std::string speciesName,
+void installMu0ThermoFromXML(const std::string& speciesName,
                              SpeciesThermo& sp, size_t k,
                              const XML_Node* Mu0Node_ptr);
 }
diff --git a/include/cantera/thermo/PDSS.h b/include/cantera/thermo/PDSS.h
index ccd457a55..0bde77843 100644
--- a/include/cantera/thermo/PDSS.h
+++ b/include/cantera/thermo/PDSS.h
@@ -281,7 +281,7 @@ private:
     /*!
      * @param msg    Message string for this error
      */
-    void err(std::string msg) const;
+    void err(const std::string& msg) const;
 
 public:
 
@@ -605,7 +605,7 @@ public:
      *                   phase. If none is given, the first XML
      *                   phase element will be used.
      */
-    virtual void initThermoXML(const XML_Node& phaseNode, std::string& id);
+    virtual void initThermoXML(const XML_Node& phaseNode, const std::string& id);
 
 
     //! This utility function reports back the type of
diff --git a/include/cantera/thermo/PDSS_ConstVol.h b/include/cantera/thermo/PDSS_ConstVol.h
index a5a889ee8..a64c1be46 100644
--- a/include/cantera/thermo/PDSS_ConstVol.h
+++ b/include/cantera/thermo/PDSS_ConstVol.h
@@ -57,7 +57,7 @@ public:
      *                   file is used.
      */
     PDSS_ConstVol(VPStandardStateTP* tp, size_t spindex,
-                  std::string inputFile, std::string id = "");
+                  const std::string& inputFile, const std::string& id = "");
 
     //! Constructor that initializes the object by examining the input file
     //! of the ThermoPhase object
@@ -365,7 +365,7 @@ public:
      *                    phase element will be used.
      */
     void constructPDSSFile(VPStandardStateTP* vptp_ptr, size_t spindex,
-                           std::string inputFile, std::string id);
+                           const std::string& inputFile, const std::string& id);
 
     //!  Initialization of a PDSS object using an xml tree
     /*!
diff --git a/include/cantera/thermo/PDSS_HKFT.h b/include/cantera/thermo/PDSS_HKFT.h
index 75c5ffe54..003f6f40e 100644
--- a/include/cantera/thermo/PDSS_HKFT.h
+++ b/include/cantera/thermo/PDSS_HKFT.h
@@ -94,7 +94,7 @@ public:
      *                   file is used.
      */
     PDSS_HKFT(VPStandardStateTP* vptp_ptr, size_t spindex,
-              std::string inputFile, std::string id = "");
+              const std::string& inputFile, const std::string& id = "");
 
     //! Constructor that initializes the object by examining the input file
     //! of the ThermoPhase object
@@ -380,7 +380,7 @@ public:
      *                    phase element will be used.
      */
     void constructPDSSFile(VPStandardStateTP* vptp_ptr, size_t spindex,
-                           std::string inputFile, std::string id);
+                           const std::string& inputFile, const std::string& id);
 
     //!  Initialization of a PDSS object using an xml tree
     /*!
diff --git a/include/cantera/thermo/PDSS_IdealGas.h b/include/cantera/thermo/PDSS_IdealGas.h
index 0472491b3..771216575 100644
--- a/include/cantera/thermo/PDSS_IdealGas.h
+++ b/include/cantera/thermo/PDSS_IdealGas.h
@@ -69,7 +69,7 @@ public:
      *                   file is used.
      */
     PDSS_IdealGas(VPStandardStateTP* tp, int spindex,
-                  std::string inputFile, std::string id = "");
+                  const std::string& inputFile, const std::string& id = "");
 
 
     //! Constructor that initializes the object by examining the input file
@@ -364,7 +364,7 @@ public:
      *                    phase element will be used.
      */
     void constructPDSSFile(VPStandardStateTP* vptp_ptr, size_t spindex,
-                           std::string inputFile, std::string id);
+                           const std::string& inputFile, const std::string& id);
 
     //!Initialization of a PDSS object using an xml tree
     /*!
@@ -389,7 +389,7 @@ public:
      *                   phase element will be used.
      */
     void constructPDSSXML(VPStandardStateTP* vptp_ptr, size_t spindex,
-                          const XML_Node& phaseNode, std::string id);
+                          const XML_Node& phaseNode, const std::string& id);
 
     //! Initialization routine for the PDSS object based on the phaseNode
     /*!
diff --git a/include/cantera/thermo/PDSS_IonsFromNeutral.h b/include/cantera/thermo/PDSS_IonsFromNeutral.h
index 09a076e1f..e52b304bd 100644
--- a/include/cantera/thermo/PDSS_IonsFromNeutral.h
+++ b/include/cantera/thermo/PDSS_IonsFromNeutral.h
@@ -59,7 +59,7 @@ public:
      *                   file is used.
      */
     PDSS_IonsFromNeutral(VPStandardStateTP* tp, size_t spindex,
-                         std::string inputFile, std::string id = "");
+                         const std::string& inputFile, const std::string& id = "");
 
 
     //! Constructor that initializes the object by examining the input file
@@ -393,7 +393,7 @@ public:
      *                    phase element will be used.
      */
     void constructPDSSFile(VPStandardStateTP* vptp_ptr, size_t spindex,
-                           std::string inputFile, std::string id);
+                           const std::string& inputFile, const std::string& id);
 
     //!  Initialization of a PDSS object using an xml tree
     /*!
@@ -421,7 +421,7 @@ public:
      */
     void constructPDSSXML(VPStandardStateTP* vptp_ptr, size_t spindex,
                           const XML_Node& speciesNode,
-                          const XML_Node& phaseNode, std::string id);
+                          const XML_Node& phaseNode, const std::string& id);
 
     //! Initialization routine for the PDSS object based on the phaseNode
     /*!
diff --git a/include/cantera/thermo/PDSS_SSVol.h b/include/cantera/thermo/PDSS_SSVol.h
index 6610f678b..83c60c159 100644
--- a/include/cantera/thermo/PDSS_SSVol.h
+++ b/include/cantera/thermo/PDSS_SSVol.h
@@ -192,7 +192,7 @@ public:
      *                   file is used.
      */
     PDSS_SSVol(VPStandardStateTP* tp, size_t spindex,
-               std::string inputFile, std::string id = "");
+               const std::string& inputFile, const std::string& id = "");
 
     //! Constructor that initializes the object by examining the input file
     //! of the ThermoPhase object
@@ -508,7 +508,7 @@ private:
      *                    phase element will be used.
      */
     void constructPDSSFile(VPStandardStateTP* vptp_ptr, size_t spindex,
-                           std::string inputFile, std::string id);
+                           const std::string& inputFile, const std::string& id);
 
     //!  Initialization of a PDSS object using an xml tree
     /*!
diff --git a/include/cantera/thermo/PDSS_Water.h b/include/cantera/thermo/PDSS_Water.h
index b949dda5c..2a71ab99a 100644
--- a/include/cantera/thermo/PDSS_Water.h
+++ b/include/cantera/thermo/PDSS_Water.h
@@ -106,7 +106,7 @@ public:
      *                   file is used.
      */
     PDSS_Water(VPStandardStateTP* tp, int spindex,
-               std::string inputFile, std::string id = "");
+               const std::string& inputFile, const std::string& id = "");
 
     //! Constructor that initializes the object by examining the input file
     //! of the variable pressure ThermoPhase object
@@ -430,7 +430,7 @@ public:
      *                    phase element will be used.
      */
     void constructPDSSFile(VPStandardStateTP* vptp_ptr, int spindex,
-                           std::string inputFile, std::string id);
+                           const std::string& inputFile, const std::string& id);
 
     //!Initialization of a PDSS object using an xml tree
     /*!
@@ -455,7 +455,7 @@ public:
      *                   phase element will be used.
      */
     void constructPDSSXML(VPStandardStateTP* vptp_ptr, int spindex,
-                          const XML_Node& phaseNode, std::string id);
+                          const XML_Node& phaseNode, const std::string& id);
 
     //! Initialization routine for all of the shallow pointers
     /*!
diff --git a/include/cantera/thermo/Phase.h b/include/cantera/thermo/Phase.h
index ddf1fa8ba..18bcc985a 100644
--- a/include/cantera/thermo/Phase.h
+++ b/include/cantera/thermo/Phase.h
@@ -37,7 +37,7 @@ class ElementsFrozen : public CanteraError
 public:
     //! Constructor for class
     //! @param func Function where the error occurred.
-    ElementsFrozen(std::string func)
+    ElementsFrozen(const std::string& func)
         : CanteraError(func, "Elements cannot be added after species.") {}
 };
 
@@ -138,14 +138,14 @@ public:
 
     //! Set the string id for the phase.
     //!     @param id String id of the phase
-    void setID(std::string id);
+    void setID(const std::string& id);
 
     //! Return the name of the phase.
     std::string name() const;
 
     //! Sets the string name for the phase.
     //!     @param nm String name of the phase
-    void setName(std::string nm);
+    void setName(const std::string& nm);
     //!@} end group Name and ID
 
     //! @name Element and Species Information
@@ -159,7 +159,7 @@ public:
     //! assigned to each element in the order it was added. Returns \ref npos
     //! if the specified element is not found.
     //!     @param name Name of the element
-    size_t elementIndex(std::string name) const;
+    size_t elementIndex(const std::string& name) const;
 
     //! Return a read-only reference to the vector of element names.
     const std::vector<std::string>& elementNames() const;
@@ -233,7 +233,7 @@ public:
     //!            phaseName:speciesName
     //!     @return The index of the species. If the name is not found,
     //!             the value \ref npos is returned.
-    size_t speciesIndex(std::string name) const;
+    size_t speciesIndex(const std::string& name) const;
 
     //! Name of the species with index k
     //!     @param k index of the species
@@ -414,7 +414,7 @@ public:
     //! Return the mole fraction of a single species
     //!     @param  name  String name of the species
     //!     @return Mole fraction of the species
-    doublereal moleFraction(std::string name) const;
+    doublereal moleFraction(const std::string& name) const;
 
     //! Return the mass fraction of a single species
     //!     @param  k species index
@@ -424,7 +424,7 @@ public:
     //! Return the mass fraction of a single species
     //!     @param  name  String name of the species
     //!     @return Mass Fraction of the species
-    doublereal massFraction(std::string name) const;
+    doublereal massFraction(const std::string& name) const;
 
     //! Get the species mole fraction vector.
     //!     @param x On return, x contains the mole fractions. Must have a
diff --git a/include/cantera/thermo/PhaseCombo_Interaction.h b/include/cantera/thermo/PhaseCombo_Interaction.h
index a8693494b..352ce123a 100644
--- a/include/cantera/thermo/PhaseCombo_Interaction.h
+++ b/include/cantera/thermo/PhaseCombo_Interaction.h
@@ -369,7 +369,7 @@ public:
      * @param id        ID of the phase in the input file. Defaults to the
      *                  empty string.
      */
-    PhaseCombo_Interaction(std::string inputFile, std::string id = "");
+    PhaseCombo_Interaction(const std::string& inputFile, const std::string& id = "");
 
     //! Construct and initialize a PhaseCombo_Interaction ThermoPhase object
     //! directly from an XML database
@@ -378,7 +378,7 @@ public:
      *  @param id     id attribute containing the name of the phase.
      *                (default is the empty string)
      */
-    PhaseCombo_Interaction(XML_Node& phaseRef, std::string id = "");
+    PhaseCombo_Interaction(XML_Node& phaseRef, const std::string& id = "");
 
 
     //! Special constructor for a hard-coded problem
@@ -679,7 +679,7 @@ public:
      *             to see if phaseNode is pointing to the phase
      *             with the correct id.
      */
-    void initThermoXML(XML_Node& phaseNode, std::string id);
+    void initThermoXML(XML_Node& phaseNode, const std::string& id);
 
     /**
      * @}
@@ -832,7 +832,7 @@ private:
      *
      * @param msg  Message to be printed
      */
-    doublereal err(std::string msg) const;
+    doublereal err(const std::string& msg) const;
 
 protected:
 
diff --git a/include/cantera/thermo/PseudoBinaryVPSSTP.h b/include/cantera/thermo/PseudoBinaryVPSSTP.h
index 188779463..8813f70bb 100644
--- a/include/cantera/thermo/PseudoBinaryVPSSTP.h
+++ b/include/cantera/thermo/PseudoBinaryVPSSTP.h
@@ -311,7 +311,7 @@ public:
      *             to see if phaseNode is pointing to the phase
      *             with the correct id.
      */
-    void initThermoXML(XML_Node& phaseNode, std::string id);
+    void initThermoXML(XML_Node& phaseNode, const std::string& id);
 
 
     //! returns a summary of the state of the phase as a string
@@ -338,7 +338,7 @@ private:
      *
      * @param msg  Message to be printed
      */
-    doublereal err(std::string msg) const;
+    doublereal err(const std::string& msg) const;
 
 protected:
 
diff --git a/include/cantera/thermo/RedlichKisterVPSSTP.h b/include/cantera/thermo/RedlichKisterVPSSTP.h
index df860e786..978cb6dd3 100644
--- a/include/cantera/thermo/RedlichKisterVPSSTP.h
+++ b/include/cantera/thermo/RedlichKisterVPSSTP.h
@@ -336,7 +336,7 @@ public:
      * @param id        ID of the phase in the input file. Defaults to the
      *                  empty string.
      */
-    RedlichKisterVPSSTP(std::string inputFile, std::string id = "");
+    RedlichKisterVPSSTP(const std::string& inputFile, const std::string& id = "");
 
     //! Construct and initialize a RedlichKisterVPSSTP ThermoPhase object
     //! directly from an XML database
@@ -345,7 +345,7 @@ public:
      *  @param id     id attribute containing the name of the phase.
      *                (default is the empty string)
      */
-    RedlichKisterVPSSTP(XML_Node& phaseRef, std::string id = "");
+    RedlichKisterVPSSTP(XML_Node& phaseRef, const std::string& id = "");
 
 
     //! Special constructor for a hard-coded problem
@@ -681,7 +681,7 @@ public:
      *             to see if phaseNode is pointing to the phase
      *             with the correct id.
      */
-    void initThermoXML(XML_Node& phaseNode, std::string id);
+    void initThermoXML(XML_Node& phaseNode, const std::string& id);
 
     /**
      * @}
@@ -828,7 +828,7 @@ private:
      *
      * @param msg  Message to be printed
      */
-    doublereal err(std::string msg) const;
+    doublereal err(const std::string& msg) const;
 
 protected:
 
diff --git a/include/cantera/thermo/RedlichKwongMFTP.h b/include/cantera/thermo/RedlichKwongMFTP.h
index bcf77edf3..09d243e87 100644
--- a/include/cantera/thermo/RedlichKwongMFTP.h
+++ b/include/cantera/thermo/RedlichKwongMFTP.h
@@ -63,7 +63,7 @@ public:
      *                  to set up the object
      * @param id        ID of the phase in the input file. Defaults to the empty string.
      */
-    RedlichKwongMFTP(std::string infile, std::string id="");
+    RedlichKwongMFTP(const std::string& infile, std::string id="");
 
     //! Construct and initialize a RedlichKwongMFTP ThermoPhase object
     //! directly from an XML database
@@ -71,7 +71,7 @@ public:
      *  @param phaseRef XML phase node containing the description of the phase
      *  @param id       id attribute containing the name of the phase.  (default is the empty string)
      */
-    RedlichKwongMFTP(XML_Node& phaseRef, std::string id = "");
+    RedlichKwongMFTP(XML_Node& phaseRef, const std::string& id = "");
 
     //!  This is a special constructor, used to replicate test problems
     //!  during the initial verification of the object
@@ -551,7 +551,7 @@ public:
      *             to see if phaseNode is pointing to the phase
      *             with the correct id.
      */
-    virtual void initThermoXML(XML_Node& phaseNode, std::string id);
+    virtual void initThermoXML(XML_Node& phaseNode, const std::string& id);
 
 private:
     //! Read the pure species RedlichKwong input parameters
diff --git a/include/cantera/thermo/SimpleThermo.h b/include/cantera/thermo/SimpleThermo.h
index c61af9baa..0a3f683c5 100644
--- a/include/cantera/thermo/SimpleThermo.h
+++ b/include/cantera/thermo/SimpleThermo.h
@@ -151,7 +151,7 @@ public:
      *
      * @see ConstCpPoly
      */
-    virtual void install(std::string name, size_t index, int type, const doublereal* c,
+    virtual void install(const std::string& name, size_t index, int type, const doublereal* c,
                          doublereal minTemp, doublereal maxTemp, doublereal refPressure) {
 
         m_logt0.push_back(log(c[0]));
diff --git a/include/cantera/thermo/SingleSpeciesTP.h b/include/cantera/thermo/SingleSpeciesTP.h
index 01e1e47b9..e6a99ff2b 100644
--- a/include/cantera/thermo/SingleSpeciesTP.h
+++ b/include/cantera/thermo/SingleSpeciesTP.h
@@ -760,7 +760,7 @@ private:
     /*!
      * @param msg   String message
      */
-    doublereal err(std::string msg) const;
+    doublereal err(const std::string& msg) const;
 };
 
 }
diff --git a/include/cantera/thermo/SpeciesThermo.h b/include/cantera/thermo/SpeciesThermo.h
index 6f00c7a3f..640446a40 100644
--- a/include/cantera/thermo/SpeciesThermo.h
+++ b/include/cantera/thermo/SpeciesThermo.h
@@ -234,7 +234,7 @@ public:
      *                    parameterization.
      * @see speciesThermoTypes.h
      */
-    virtual void install(std::string name, size_t index, int type,
+    virtual void install(const std::string& name, size_t index, int type,
                          const doublereal* c,
                          doublereal minTemp, doublereal maxTemp,
                          doublereal refPressure)=0;
diff --git a/include/cantera/thermo/SpeciesThermoFactory.h b/include/cantera/thermo/SpeciesThermoFactory.h
index 46ce29680..1341cc428 100644
--- a/include/cantera/thermo/SpeciesThermoFactory.h
+++ b/include/cantera/thermo/SpeciesThermoFactory.h
@@ -37,8 +37,8 @@ public:
      * @param spName Species Name that caused the error
      * @param speciesThermoModel Unrecognized species thermo model name
      */
-    UnknownSpeciesThermoModel(std::string proc, std::string spName,
-                              std::string speciesThermoModel) :
+    UnknownSpeciesThermoModel(const std::string& proc, const std::string& spName,
+                              const std::string& speciesThermoModel) :
         CanteraError(proc, "species " + spName +
                      ": Specified speciesThermoPhase model "
                      + speciesThermoModel +
diff --git a/include/cantera/thermo/SpeciesThermoMgr.h b/include/cantera/thermo/SpeciesThermoMgr.h
index 1a4eef52d..f25104853 100644
--- a/include/cantera/thermo/SpeciesThermoMgr.h
+++ b/include/cantera/thermo/SpeciesThermoMgr.h
@@ -34,7 +34,7 @@ public:
      * @param proc   name of the procecdure
      * @param type   unknown type
      */
-    UnknownSpeciesThermo(std::string proc, int type) :
+    UnknownSpeciesThermo(const std::string& proc, int type) :
         CanteraError(proc, "Specified species parameterization type (" + int2str(type)
                      + ") does not match any known type.") {}
 
@@ -43,7 +43,7 @@ public:
      * @param proc   name of the procecdure
      * @param stype   String name for the unknown type
      */
-    UnknownSpeciesThermo(std::string proc, std::string stype) :
+    UnknownSpeciesThermo(const std::string& proc, const std::string& stype) :
         CanteraError(proc, "Specified species parameterization type (" + stype
                      + ") does not match any known type.") {}
     //! destructor
@@ -116,7 +116,7 @@ public:
      *                    parameterization.
      * @see speciesThermoTypes.h
      */
-    virtual void install(std::string name, size_t sp, int type,
+    virtual void install(const std::string& name, size_t sp, int type,
                          const doublereal* c,
                          doublereal minTemp, doublereal maxTemp,
                          doublereal refPressure);
@@ -277,7 +277,7 @@ SpeciesThermoDuo<T1, T2>::duplMyselfAsSpeciesThermo() const
 
 template<class T1, class T2>
 void
-SpeciesThermoDuo<T1, T2>::install(std::string name, size_t sp, int type,
+SpeciesThermoDuo<T1, T2>::install(const std::string& name, size_t sp, int type,
                                   const doublereal* c,
                                   doublereal minTemp,
                                   doublereal maxTemp,
diff --git a/include/cantera/thermo/StatMech.h b/include/cantera/thermo/StatMech.h
index 26df38b32..13e58ba21 100644
--- a/include/cantera/thermo/StatMech.h
+++ b/include/cantera/thermo/StatMech.h
@@ -44,7 +44,7 @@ public:
      *                     parameters for the standard state.
      */
     StatMech(int n, doublereal tlow, doublereal thigh, doublereal pref,
-             const doublereal* coeffs, std::string my_name);
+             const doublereal* coeffs, const std::string& my_name);
 
     //! copy constructor
     /*!
diff --git a/include/cantera/thermo/StoichSubstanceSSTP.h b/include/cantera/thermo/StoichSubstanceSSTP.h
index f32f49a03..3759c4cdf 100644
--- a/include/cantera/thermo/StoichSubstanceSSTP.h
+++ b/include/cantera/thermo/StoichSubstanceSSTP.h
@@ -173,7 +173,7 @@ public:
      * @param id     name of the phase id in the file.
      *               If this is blank, the first phase in the file is used.
      */
-    StoichSubstanceSSTP(std::string infile, std::string id = "");
+    StoichSubstanceSSTP(const std::string& infile, std::string id = "");
 
     //! Construct and initialize a StoichSubstanceSSTP ThermoPhase object
     //! directly from an XML database
@@ -181,7 +181,7 @@ public:
      *  @param phaseRef XML node pointing to a StoichSubstanceSSTP description
      *  @param id       Id of the phase.
      */
-    StoichSubstanceSSTP(XML_Node& phaseRef, std::string id = "");
+    StoichSubstanceSSTP(XML_Node& phaseRef, const std::string& id = "");
 
     //! Copy constructor
     /*!
@@ -466,7 +466,7 @@ public:
     virtual void initThermo();
 
 
-    virtual void initThermoXML(XML_Node& phaseNode, std::string id);
+    virtual void initThermoXML(XML_Node& phaseNode, const std::string& id);
 
     //! Set the equation of state parameters
     /*!
@@ -544,7 +544,7 @@ public:
      * @param id     name of the phase id in the file.
      *               If this is blank, the first phase in the file is used.
      */
-    electrodeElectron(std::string infile, std::string id = "");
+    electrodeElectron(const std::string& infile, std::string id = "");
 
     //! Construct and initialize a electrodeElectron ThermoPhase object
     //! directly from an XML database
@@ -552,7 +552,7 @@ public:
      *  @param phaseRef XML node pointing to a electrodeElectron description
      *  @param id       Id of the phase.
      */
-    electrodeElectron(XML_Node& phaseRef, std::string id = "");
+    electrodeElectron(XML_Node& phaseRef, const std::string& id = "");
 
     //! Copy constructor
     /*!
@@ -571,7 +571,7 @@ public:
 
     void setParametersFromXML(const XML_Node& eosdata);
 
-    virtual void initThermoXML(XML_Node& phaseNode, std::string id);
+    virtual void initThermoXML(XML_Node& phaseNode, const std::string& id);
 
     void setParameters(int n, doublereal* const c);
 };
diff --git a/include/cantera/thermo/SurfPhase.h b/include/cantera/thermo/SurfPhase.h
index 6a653c6d8..99bac2b48 100644
--- a/include/cantera/thermo/SurfPhase.h
+++ b/include/cantera/thermo/SurfPhase.h
@@ -160,7 +160,7 @@ public:
      * @param id     name of the phase id in the file.
      *               If this is blank, the first phase in the file is used.
      */
-    SurfPhase(std::string infile, std::string id);
+    SurfPhase(const std::string& infile, std::string id);
 
     //! Construct and initialize a SurfPhase ThermoPhase object
     //! directly from an XML database
@@ -602,7 +602,7 @@ public:
     /*!
      *  @param cov  String containing colon-separated  name:value pairs
      */
-    void setCoveragesByName(std::string cov);
+    void setCoveragesByName(const std::string& cov);
 
     //! Return a vector of surface coverages
     /*!
diff --git a/include/cantera/thermo/ThermoFactory.h b/include/cantera/thermo/ThermoFactory.h
index 79720734e..63b31c108 100644
--- a/include/cantera/thermo/ThermoFactory.h
+++ b/include/cantera/thermo/ThermoFactory.h
@@ -42,7 +42,8 @@ public:
      * @param proc Function name where the error occurred.
      * @param thermoModel Sting name of ThermoPhase which didn't match
      */
-    UnknownThermoPhaseModel(std::string proc, std::string thermoModel) :
+    UnknownThermoPhaseModel(const std::string& proc,
+                            const std::string& thermoModel) :
         CanteraError(proc, "Specified ThermoPhase model "
                      + thermoModel +
                      " does not match any known type.") {}
@@ -97,7 +98,7 @@ public:
      *   Throws an exception UnknownThermoPhaseModel if the string
      *   wasn't matched.
      */
-    virtual ThermoPhase* newThermoPhase(std::string model);
+    virtual ThermoPhase* newThermoPhase(const std::string& model);
 
 private:
     //! static member of a single instance
@@ -121,7 +122,7 @@ private:
  *   Throws an exception UnknownThermoPhaseModel if the string
  *   wasn't matched.
  */
-inline ThermoPhase* newThermoPhase(std::string model,
+inline ThermoPhase* newThermoPhase(const std::string& model,
                                    ThermoFactory* f=0)
 {
     if (f == 0) {
@@ -175,7 +176,7 @@ ThermoPhase* newPhase(XML_Node& phase);
  * @return
  *   Returns an initialized ThermoPhase object.
  */
-ThermoPhase* newPhase(std::string infile, std::string id);
+ThermoPhase* newPhase(const std::string& infile, std::string id);
 
 //! Import a phase information into an empty thermophase object
 /*!
@@ -304,7 +305,7 @@ bool installSpecies(size_t k, const XML_Node& s, thermo_t& p,
  *
  *
  */
-const XML_Node* speciesXML_Node(std::string kname,
+const XML_Node* speciesXML_Node(const std::string& kname,
                                 const XML_Node* phaseSpeciesData);
 
 //@}
diff --git a/include/cantera/thermo/ThermoPhase.h b/include/cantera/thermo/ThermoPhase.h
index b1727c48d..63c4e37e2 100644
--- a/include/cantera/thermo/ThermoPhase.h
+++ b/include/cantera/thermo/ThermoPhase.h
@@ -1385,8 +1385,8 @@ public:
      *            phase. If none is given, the first XML
      *            phase element encountered will be used.
      */
-    virtual void initThermoFile(std::string inputFile, std::string id);
-
+    virtual void initThermoFile(const std::string& inputFile,
+                                const std::string& id);
 
     //!Import and initialize a ThermoPhase object  using an XML tree.
     /*!
@@ -1418,7 +1418,7 @@ public:
      *             to see if phaseNode is pointing to the phase
      *             with the correct id.
      */
-    virtual void initThermoXML(XML_Node& phaseNode, std::string id);
+    virtual void initThermoXML(XML_Node& phaseNode, const std::string& id);
 
     //! Initialize the ThermoPhase object after all species have been set up
     /*!
@@ -1676,7 +1676,7 @@ private:
     /*!
      * @param msg String containing the message.
      */
-    doublereal err(std::string msg) const;
+    doublereal err(const std::string& msg) const;
 
 };
 
diff --git a/include/cantera/thermo/VPSSMgr.h b/include/cantera/thermo/VPSSMgr.h
index c6d25ee6c..141229f25 100644
--- a/include/cantera/thermo/VPSSMgr.h
+++ b/include/cantera/thermo/VPSSMgr.h
@@ -738,7 +738,7 @@ public:
      *  @param phaseNode   Reference to the phaseNode XML node.
      *  @param id          ID of the phase.
      */
-    virtual void initThermoXML(XML_Node& phaseNode, std::string id);
+    virtual void initThermoXML(XML_Node& phaseNode, const std::string& id);
 
     //! Install specific content for species k in the reference-state
     //! thermodynamic SpeciesManager object
@@ -962,7 +962,7 @@ private:
     /*!
      * @param msg  Error message string
      */
-    void err(std::string msg) const;
+    void err(const std::string& msg) const;
 
 };
 //@}
diff --git a/include/cantera/thermo/VPSSMgr_ConstVol.h b/include/cantera/thermo/VPSSMgr_ConstVol.h
index 6f4669f04..7c518ccc4 100644
--- a/include/cantera/thermo/VPSSMgr_ConstVol.h
+++ b/include/cantera/thermo/VPSSMgr_ConstVol.h
@@ -178,7 +178,7 @@ public:
      *  @param phaseNode   Reference to the phase node in the XML tree
      *  @param id          string name of the phase
      */
-    virtual void initThermoXML(XML_Node& phaseNode, std::string id);
+    virtual void initThermoXML(XML_Node& phaseNode, const std::string& id);
 
     //!  Create and install a constant volume pressure dependent
     //!  standard state for one species within this object
diff --git a/include/cantera/thermo/VPSSMgr_General.h b/include/cantera/thermo/VPSSMgr_General.h
index 03a62c2bb..fc321865d 100644
--- a/include/cantera/thermo/VPSSMgr_General.h
+++ b/include/cantera/thermo/VPSSMgr_General.h
@@ -197,7 +197,7 @@ protected:
      *  @param phaseNode   Reference to the phaseNode XML node.
      *  @param id          ID of the phase.
      */
-    virtual void initThermoXML(XML_Node& phaseNode, std::string id);
+    virtual void initThermoXML(XML_Node& phaseNode, const std::string& id);
 
 private:
     //! Local factory routine for the creation of PDSS objects
diff --git a/include/cantera/thermo/VPSSMgr_IdealGas.h b/include/cantera/thermo/VPSSMgr_IdealGas.h
index b45e82998..4540a86c3 100644
--- a/include/cantera/thermo/VPSSMgr_IdealGas.h
+++ b/include/cantera/thermo/VPSSMgr_IdealGas.h
@@ -178,7 +178,7 @@ public:
      *  @param phaseNode   Reference to the phase node in the XML tree
      *  @param id          string name of the phase
      */
-    virtual void initThermoXML(XML_Node& phaseNode, std::string id);
+    virtual void initThermoXML(XML_Node& phaseNode, const std::string& id);
 
     //!  Create and install an ideal gas standard state manager
     //!  for one species within this object
diff --git a/include/cantera/thermo/VPSSMgr_Water_ConstVol.h b/include/cantera/thermo/VPSSMgr_Water_ConstVol.h
index ca55e0884..bd09f7386 100644
--- a/include/cantera/thermo/VPSSMgr_Water_ConstVol.h
+++ b/include/cantera/thermo/VPSSMgr_Water_ConstVol.h
@@ -263,7 +263,7 @@ public:
      *  @param phaseNode   Reference to the phaseNode XML node.
      *  @param id          ID of the phase.
      */
-    virtual void initThermoXML(XML_Node& phaseNode, std::string id);
+    virtual void initThermoXML(XML_Node& phaseNode, const std::string& id);
 
     //! Install specific content for species k in the standard-state
     //! thermodynamic calculator and also create/return a PDSS object
diff --git a/include/cantera/thermo/VPSSMgr_Water_HKFT.h b/include/cantera/thermo/VPSSMgr_Water_HKFT.h
index 38b7c392e..48582890e 100644
--- a/include/cantera/thermo/VPSSMgr_Water_HKFT.h
+++ b/include/cantera/thermo/VPSSMgr_Water_HKFT.h
@@ -347,7 +347,7 @@ public:
      *  @param phaseNode   Reference to the phaseNode XML node.
      *  @param id          ID of the phase.
      */
-    virtual void initThermoXML(XML_Node& phaseNode, std::string id);
+    virtual void initThermoXML(XML_Node& phaseNode, const std::string& id);
 
     //! Install specific content for species k in the standard-state
     //! thermodynamic calculator and also create/return a PDSS object
diff --git a/include/cantera/thermo/VPStandardStateTP.h b/include/cantera/thermo/VPStandardStateTP.h
index cccdeae65..b584ccec3 100644
--- a/include/cantera/thermo/VPStandardStateTP.h
+++ b/include/cantera/thermo/VPStandardStateTP.h
@@ -565,7 +565,7 @@ public:
      *             to see if phaseNode is pointing to the phase
      *             with the correct id.
      */
-    virtual void initThermoXML(XML_Node& phaseNode, std::string id);
+    virtual void initThermoXML(XML_Node& phaseNode, const std::string& id);
 
 
     //! set the VPSS Mgr
@@ -640,7 +640,7 @@ private:
     /*!
      * @param msg  Error message string
      */
-    doublereal err(std::string msg) const;
+    doublereal err(const std::string& msg) const;
 
 };
 }
diff --git a/include/cantera/thermo/WaterSSTP.h b/include/cantera/thermo/WaterSSTP.h
index 13ccd7ab0..a17f7f136 100644
--- a/include/cantera/thermo/WaterSSTP.h
+++ b/include/cantera/thermo/WaterSSTP.h
@@ -142,14 +142,14 @@ public:
      * @param inputFile String name of the input file
      * @param id        string id of the phase name
      */
-    WaterSSTP(std::string inputFile, std::string id = "");
+    explicit WaterSSTP(const std::string& inputFile, const std::string& id = "");
 
     //! Full constructor for a water phase
     /*!
      * @param phaseRef  XML node referencing the water phase.
      * @param id        string id of the phase name
      */
-    WaterSSTP(XML_Node& phaseRef, std::string id = "");
+    explicit WaterSSTP(XML_Node& phaseRef, const std::string& id = "");
 
     //! Destructor
     virtual ~WaterSSTP();
@@ -442,7 +442,7 @@ public:
      *             to see if phaseNode is pointing to the phase
      *             with the correct id.
      */
-    virtual void initThermoXML(XML_Node& phaseNode, std::string id);
+    virtual void initThermoXML(XML_Node& phaseNode, const std::string& id);
 
     //! Initialize the ThermoPhase object after all species have been set up
     /*!
diff --git a/include/cantera/tpx/Sub.h b/include/cantera/tpx/Sub.h
index 210cfa09b..3039e4c6e 100644
--- a/include/cantera/tpx/Sub.h
+++ b/include/cantera/tpx/Sub.h
@@ -30,7 +30,8 @@ namespace tpx
 class TPX_Error : public Cantera::CanteraError
 {
 public:
-    TPX_Error(std::string p, std::string e) : CanteraError(p, e) { }
+    TPX_Error(const std::string& p, const std::string& e) :
+        CanteraError(p, e) { }
 };
 
 namespace PropertyPair
diff --git a/include/cantera/transport/LTPspecies.h b/include/cantera/transport/LTPspecies.h
index abdaba271..b78f7dc31 100644
--- a/include/cantera/transport/LTPspecies.h
+++ b/include/cantera/transport/LTPspecies.h
@@ -97,7 +97,7 @@ public:
      *                          is creating a parameterization for (e.g., viscosity)
      *   @param   thermo        const pointer to the ThermoPhase object, which is used to find the temperature.
      */
-    LTPspecies(const XML_Node* const propNode = 0,  std::string name = "-",
+    LTPspecies(const XML_Node* const propNode = 0,  const std::string& name = "-",
                TransportPropertyType tp_ind = TP_UNKNOWN, const thermo_t* thermo = 0);
 
     //! Copy constructor
@@ -225,7 +225,7 @@ public:
      *                          is creating a parameterization for (e.g., viscosity)
      *   @param   thermo        const pointer to the ThermoPhase object, which is used to find the temperature.
      */
-    LTPspecies_Const(const XML_Node& propNode,  std::string name,
+    LTPspecies_Const(const XML_Node& propNode,  const std::string& name,
                      TransportPropertyType tp_ind,  const thermo_t* const thermo);
 
     //! Copy constructor
@@ -309,7 +309,7 @@ public:
      *   @param   thermo        const pointer to the ThermoPhase object, which is used to find the temperature.
      *
      */
-    LTPspecies_Arrhenius(const XML_Node& propNode, std::string name,
+    LTPspecies_Arrhenius(const XML_Node& propNode, const std::string& name,
                          TransportPropertyType tp_ind, const thermo_t* thermo);
 
     //! Copy constructor
@@ -416,7 +416,7 @@ public:
      *   @param   thermo        const pointer to the ThermoPhase object, which is used to find the temperature.
      *
      */
-    LTPspecies_Poly(const XML_Node& propNode, std::string name, TransportPropertyType tp_ind, const thermo_t* thermo);
+    LTPspecies_Poly(const XML_Node& propNode, const std::string& name, TransportPropertyType tp_ind, const thermo_t* thermo);
 
     //! Copy constructor
     /*!
@@ -504,7 +504,7 @@ public:
      *   @param   thermo        const pointer to the ThermoPhase object, which is used to find the temperature.
      *
      */
-    LTPspecies_ExpT(const XML_Node& propNode,  std::string name,
+    LTPspecies_ExpT(const XML_Node& propNode, const std::string& name,
                     TransportPropertyType tp_ind,  const thermo_t* thermo);
 
     //! Copy constructor
diff --git a/include/cantera/transport/LiquidTransport.h b/include/cantera/transport/LiquidTransport.h
index 7fe6a2453..99b73f117 100644
--- a/include/cantera/transport/LiquidTransport.h
+++ b/include/cantera/transport/LiquidTransport.h
@@ -1430,7 +1430,7 @@ private:
      *
      * @param msg      Indicates the member function which is not implemented
      */
-    doublereal err(std::string msg) const;
+    doublereal err(const std::string& msg) const;
 
 };
 }
diff --git a/include/cantera/transport/SimpleTransport.h b/include/cantera/transport/SimpleTransport.h
index eb2012af7..6354bf8e9 100644
--- a/include/cantera/transport/SimpleTransport.h
+++ b/include/cantera/transport/SimpleTransport.h
@@ -871,7 +871,7 @@ private:
      *
      * @param msg   Indicates the member function which is not implemented
      */
-    doublereal err(std::string msg) const;
+    doublereal err(const std::string& msg) const;
 
 };
 }
diff --git a/include/cantera/transport/TransportBase.h b/include/cantera/transport/TransportBase.h
index 6aa0bf272..5e424441d 100644
--- a/include/cantera/transport/TransportBase.h
+++ b/include/cantera/transport/TransportBase.h
@@ -887,7 +887,7 @@ private:
      *
      *  @return  returns a double, though we will never get there
      */
-    doublereal err(std::string msg) const;
+    doublereal err(const std::string& msg) const;
 
 };
 
diff --git a/include/cantera/transport/TransportFactory.h b/include/cantera/transport/TransportFactory.h
index f40c32410..f4febbdcf 100644
--- a/include/cantera/transport/TransportFactory.h
+++ b/include/cantera/transport/TransportFactory.h
@@ -117,7 +117,7 @@ public:
      *  @param thermo    ThermoPhase object
      *  @param log_level log level
      */
-    virtual Transport* newTransport(std::string model, thermo_t* thermo, int log_level=0);
+    virtual Transport* newTransport(const std::string& model, thermo_t* thermo, int log_level=0);
 
     //! Build a new transport manager using the default transport manager
     //! in the phase description and return a base class pointer to it
@@ -387,7 +387,7 @@ private:
  *
  * @ingroup transportProps
  */
-Transport* newTransportMgr(std::string transportModel = "",  thermo_t* thermo = 0, int loglevel = 0,
+Transport* newTransportMgr(const std::string& transportModel = "",  thermo_t* thermo = 0, int loglevel = 0,
                            TransportFactory* f = 0);
 //====================================================================================================================
 //!  Create a new transport manager instance.
diff --git a/include/cantera/transport/TransportParams.h b/include/cantera/transport/TransportParams.h
index e026148a6..0f7b88dde 100644
--- a/include/cantera/transport/TransportParams.h
+++ b/include/cantera/transport/TransportParams.h
@@ -32,7 +32,7 @@ public:
     /*!
      *  @param method Single string indicating a method that is not implemented
      */
-    NotImplemented(std::string method);
+    NotImplemented(const std::string& method);
 };
 //====================================================================================================================
 //! Base structure to hold transport model parameters.
diff --git a/include/cantera/zeroD/FlowDevice.h b/include/cantera/zeroD/FlowDevice.h
index 09b730d42..a03e71807 100644
--- a/include/cantera/zeroD/FlowDevice.h
+++ b/include/cantera/zeroD/FlowDevice.h
@@ -175,7 +175,7 @@ private:
     ReactorBase* m_out;
     std::vector<size_t> m_in2out, m_out2in;
 
-    void warn(std::string meth) {
+    void warn(const std::string& meth) {
         writelog(std::string("Warning: method ") + meth + " of base class "
                  + " FlowDevice called. Nothing done.\n");
     }
diff --git a/include/cantera/zeroD/ReactorFactory.h b/include/cantera/zeroD/ReactorFactory.h
index c9d8dd823..373c6536b 100644
--- a/include/cantera/zeroD/ReactorFactory.h
+++ b/include/cantera/zeroD/ReactorFactory.h
@@ -45,7 +45,7 @@ public:
      * @param n the type to be created.
      */
     virtual ReactorBase* newReactor(int n);
-    virtual ReactorBase* newReactor(std::string reactorType);
+    virtual ReactorBase* newReactor(const std::string& reactorType);
 
 private:
     static ReactorFactory* s_factory;
@@ -53,7 +53,7 @@ private:
     ReactorFactory() {}
 };
 
-inline ReactorBase* newReactor(std::string model,
+inline ReactorBase* newReactor(const std::string& model,
                                ReactorFactory* f=0)
 {
     if (f == 0) {
diff --git a/samples/cxx/kinetics1/example_utils.h b/samples/cxx/kinetics1/example_utils.h
index 6dfbbdd4a..6eeed7526 100644
--- a/samples/cxx/kinetics1/example_utils.h
+++ b/samples/cxx/kinetics1/example_utils.h
@@ -47,7 +47,8 @@ void makeDataLabels(const G& gas, V& names)
 }
 
 template<class G, class A>
-void plotSoln(std::string fname, std::string fmt, std::string title, const G& gas, const A& soln)
+void plotSoln(const std::string& fname, const std::string& fmt,
+              const std::string& title, const G& gas, const A& soln)
 {
     std::vector<std::string> names;
     makeDataLabels(gas, names);
diff --git a/src/base/application.cpp b/src/base/application.cpp
index 3d056e5ba..f1f8aa9fa 100644
--- a/src/base/application.cpp
+++ b/src/base/application.cpp
@@ -112,7 +112,7 @@ Application::Messages::~Messages()
 }
 
 // Set an error condition in the application class without throwing an exception
-void Application::Messages::addError(std::string r, std::string msg)
+void Application::Messages::addError(const std::string& r, const std::string& msg)
 {
     errorMessage.push_back(msg);
     errorRoutine.push_back(r);
@@ -162,7 +162,7 @@ void Application::Messages::writelogendl()
 
 #ifdef WITH_HTML_LOGS
 
-void Application::Messages::beginLogGroup(std::string title, int _loglevel /*=-99*/)
+void Application::Messages::beginLogGroup(const std::string& title, int _loglevel /*=-99*/)
 {
     // Add the current loglevel to the vector of loglevels
     loglevels.push_back(loglevel);
@@ -192,35 +192,35 @@ void Application::Messages::beginLogGroup(std::string title, int _loglevel /*=-9
     current = &current->addChild("ul");
 }
 
-void Application::Messages::addLogEntry(std::string tag, std::string value)
+void Application::Messages::addLogEntry(const std::string& tag, const std::string& value)
 {
     if (loglevel > 0 && current) {
         current->addChild("li",tag+": "+value);
     }
 }
 
-void Application::Messages::addLogEntry(std::string tag, doublereal value)
+void Application::Messages::addLogEntry(const std::string& tag, doublereal value)
 {
     if (loglevel > 0 && current) {
         current->addChild("li",tag+": "+fp2str(value));
     }
 }
 
-void Application::Messages::addLogEntry(std::string tag, int value)
+void Application::Messages::addLogEntry(const std::string& tag, int value)
 {
     if (loglevel > 0 && current) {
         current->addChild("li",tag+": "+int2str(value));
     }
 }
 
-void Application::Messages::addLogEntry(std::string msg)
+void Application::Messages::addLogEntry(const std::string& msg)
 {
     if (loglevel > 0 && current) {
         current->addChild("li",msg);
     }
 }
 
-void Application::Messages::endLogGroup(std::string title)
+void Application::Messages::endLogGroup(const std::string& title)
 {
      if (title != "" && title != loggroups.back()) {
         writelog("Logfile error."
@@ -249,7 +249,7 @@ void Application::Messages::endLogGroup(std::string title)
     loglevels.pop_back();
 }
 
-void Application::Messages::write_logfile(std::string file)
+void Application::Messages::write_logfile(const std::string& file)
 {
     if (!xmllog) {
         return;
@@ -391,7 +391,7 @@ void Application::thread_complete()
 }
 
 
-XML_Node* Application::get_XML_File(std::string file, int debug)
+XML_Node* Application::get_XML_File(const std::string& file, int debug)
 {
     ScopedLock xmlLock(xml_mutex);
     std::string path = "";
@@ -486,7 +486,7 @@ XML_Node* Application::get_XML_File(std::string file, int debug)
 }
 
 
-void Application::close_XML_File(std::string file)
+void Application::close_XML_File(const std::string& file)
 {
     ScopedLock xmlLock(xml_mutex);
     if (file == "all") {
@@ -653,7 +653,7 @@ void Application::setDefaultDirectories()
 #endif
 }
 
-void Application::addDataDirectory(std::string dir)
+void Application::addDataDirectory(const std::string& dir)
 {
     ScopedLock dirLock(dir_mutex);
     if (inputDirs.size() == 0) {
@@ -672,7 +672,7 @@ void Application::addDataDirectory(std::string dir)
     inputDirs.push_back(d);
 }
 
-std::string Application::findInputFile(std::string name)
+std::string Application::findInputFile(const std::string& name)
 {
     ScopedLock dirLock(dir_mutex);
     string::size_type islash = name.find('/');
diff --git a/src/base/application.h b/src/base/application.h
index 6bb3a5fb1..73c5a6009 100644
--- a/src/base/application.h
+++ b/src/base/application.h
@@ -81,7 +81,7 @@ protected:
          * @param msg  Description of the error
          * @ingroup errorhandling
          */
-        void addError(std::string r, std::string msg);
+        void addError(const std::string& r, const std::string& msg);
 
         //! Return the number of errors that have been encountered so far.
         /*!
@@ -200,7 +200,7 @@ protected:
          *  @param loglevel loglevel of the group.
          *  @ingroup HTML_logs
          */
-        void beginLogGroup(std::string title, int loglevel) ;
+        void beginLogGroup(const std::string& title, int loglevel) ;
 
         //! Add an entry to an HTML log file.
         /*!
@@ -211,7 +211,7 @@ protected:
          *
          * @ingroup HTML_logs
          */
-        void addLogEntry(std::string tag, std::string value) ;
+        void addLogEntry(const std::string& tag, const std::string& value) ;
 
         //! Add an entry to an HTML log file.
         /*!
@@ -222,7 +222,7 @@ protected:
          *
          * @ingroup HTML_logs
          */
-        void addLogEntry(std::string tag, doublereal value) ;
+        void addLogEntry(const std::string& tag, doublereal value) ;
 
         //! Add an entry to an HTML log file.
         /*!
@@ -233,7 +233,7 @@ protected:
          *
          * @ingroup HTML_logs
          */
-        void addLogEntry(std::string tag, int value) ;
+        void addLogEntry(const std::string& tag, int value) ;
 
         //! Add an entry to an HTML log file.
         /*!
@@ -243,7 +243,7 @@ protected:
          *
          * @ingroup HTML_logs
          */
-        void addLogEntry(std::string msg) ;
+        void addLogEntry(const std::string& msg) ;
 
         //! Close the current group of log messages.
         /*!
@@ -257,7 +257,7 @@ protected:
          * @param title Name of the log group. It defaults to the most recent
          *              log group created.
          */
-        void endLogGroup(std::string title) ;
+        void endLogGroup(const std::string& title) ;
 
         //! Write the HTML log file.
         /*!
@@ -273,7 +273,7 @@ protected:
          *
          *  @param  file Name of the file to be written
          */
-        void write_logfile(std::string file);
+        void write_logfile(const std::string& file);
 #endif
 
     protected:
@@ -366,7 +366,7 @@ public:
      * @param msg  Description of the error
      * @ingroup errorhandling
      */
-    void addError(std::string r, std::string msg) {
+    void addError(const std::string& r, const std::string& msg) {
         pMessenger->addError(r, msg) ;
     }
 
@@ -438,7 +438,7 @@ public:
      *
      * @param dir  String name for the directory to be added to the search path
      */
-    void addDataDirectory(std::string dir) ;
+    void addDataDirectory(const std::string& dir) ;
 
     //! Find an input file.
     /*!
@@ -467,7 +467,7 @@ public:
      *
      * @ingroup inputfiles
      */
-    std::string findInputFile(std::string name) ;
+    std::string findInputFile(const std::string& name) ;
 
     //! Return a pointer to the XML tree for a Cantera input file.
     /*!
@@ -479,7 +479,7 @@ public:
      * @param file String containing the relative or absolute file name
      * @param debug Debug flag
      */
-    XML_Node* get_XML_File(std::string file, int debug=0) ;
+    XML_Node* get_XML_File(const std::string& file, int debug=0) ;
 
     //! Close an XML File
     /*!
@@ -487,7 +487,7 @@ public:
      *
      * @param file String containing the relative or absolute file name
      */
-    void close_XML_File(std::string file) ;
+    void close_XML_File(const std::string& file) ;
 
     //!  Write a message to the screen.
     /*!
@@ -573,7 +573,7 @@ public:
      *  @param loglevel loglevel of the group.
      *  @ingroup HTML_logs
      */
-    void beginLogGroup(std::string title, int loglevel) {
+    void beginLogGroup(const std::string& title, int loglevel) {
         pMessenger->beginLogGroup(title,loglevel);
     }
 
@@ -586,7 +586,7 @@ public:
      *
      * @ingroup HTML_logs
      */
-    void addLogEntry(std::string tag, std::string value) {
+    void addLogEntry(const std::string& tag, const std::string& value) {
         pMessenger->addLogEntry(tag, value);
     }
 
@@ -599,7 +599,7 @@ public:
      *
      * @ingroup HTML_logs
      */
-    void addLogEntry(std::string tag, doublereal value) {
+    void addLogEntry(const std::string& tag, doublereal value) {
         pMessenger->addLogEntry(tag, value);
     }
 
@@ -612,7 +612,7 @@ public:
      *
      * @ingroup HTML_logs
      */
-    void addLogEntry(std::string tag, int value) {
+    void addLogEntry(const std::string& tag, int value) {
         pMessenger->addLogEntry(tag, value);
     }
 
@@ -624,7 +624,7 @@ public:
      *
      * @ingroup HTML_logs
      */
-    void addLogEntry(std::string msg) {
+    void addLogEntry(const std::string& msg) {
         pMessenger->addLogEntry(msg);
     }
 
@@ -641,7 +641,7 @@ public:
      *              log group created.
      * @ingroup HTML_logs
      */
-    void endLogGroup(std::string title) {
+    void endLogGroup(const std::string& title) {
         pMessenger->endLogGroup(title) ;
     }
 
@@ -659,7 +659,7 @@ public:
      *
      *  @param  file Name of the file to be written
      */
-    void write_logfile(std::string file) {
+    void write_logfile(const std::string& file) {
         pMessenger->write_logfile(file) ;
     }
 #endif
diff --git a/src/base/ctexceptions.cpp b/src/base/ctexceptions.cpp
index 9cf66bcc2..3b3d60f6d 100644
--- a/src/base/ctexceptions.cpp
+++ b/src/base/ctexceptions.cpp
@@ -14,7 +14,7 @@ namespace Cantera
 
 static const char* stars = "***********************************************************************\n";
 
-CanteraError::CanteraError(std::string procedure, std::string msg) :
+CanteraError::CanteraError(const std::string& procedure, const std::string& msg) :
     procedure_(procedure),
     msg_(msg),
     saved_(false)
@@ -23,7 +23,7 @@ CanteraError::CanteraError(std::string procedure, std::string msg) :
     save();
 }
 
-CanteraError::CanteraError(std::string procedure) :
+CanteraError::CanteraError(const std::string& procedure) :
     procedure_(procedure),
     saved_(false)
 {
diff --git a/src/base/ctml.cpp b/src/base/ctml.cpp
index d234d8b7a..976409d8c 100644
--- a/src/base/ctml.cpp
+++ b/src/base/ctml.cpp
@@ -659,8 +659,10 @@ bool getOptionalFloat(const Cantera::XML_Node& parent,
  *
  * @return returns true if the child element named "name" exists
  */
-doublereal getFloatDefaultUnits(const Cantera::XML_Node& parent, std::string name,
-                                std::string defaultUnits, std::string type)
+doublereal getFloatDefaultUnits(const Cantera::XML_Node& parent,
+                                const std::string& name,
+                                const std::string& defaultUnits,
+                                const std::string& type)
 {
 
     doublereal fctr = 1.0;
@@ -755,7 +757,7 @@ bool getOptionalModel(const Cantera::XML_Node& parent, const std::string nodeNam
  *   @param parent reference to the XML_Node object of the parent XML element
  *   @param name   Name of the XML child element
  */
-int getInteger(const Cantera::XML_Node& parent, std::string name)
+int getInteger(const Cantera::XML_Node& parent, const std::string& name)
 {
     if (!parent.hasChild(name)) {
         throw CanteraError("getInteger (called from XML Node \"" +
diff --git a/src/base/global.cpp b/src/base/global.cpp
index cb4aff2a4..deb995f3d 100644
--- a/src/base/global.cpp
+++ b/src/base/global.cpp
@@ -73,37 +73,37 @@ void error(const std::string& msg)
 
 #ifdef WITH_HTML_LOGS
 
-void beginLogGroup(std::string title, int loglevel)
+void beginLogGroup(const std::string& title, int loglevel)
 {
     app()->beginLogGroup(title, loglevel) ;
 }
 
-void addLogEntry(std::string tag, std::string value)
+void addLogEntry(const std::string& tag, const std::string& value)
 {
     app()->addLogEntry(tag, value) ;
 }
 
-void addLogEntry(std::string tag, doublereal value)
+void addLogEntry(const std::string& tag, doublereal value)
 {
     app()->addLogEntry(tag, value) ;
 }
 
-void addLogEntry(std::string tag, int value)
+void addLogEntry(const std::string& tag, int value)
 {
     app()->addLogEntry(tag, value) ;
 }
 
-void addLogEntry(std::string msg)
+void addLogEntry(const std::string& msg)
 {
     app()->addLogEntry(msg) ;
 }
 
-void endLogGroup(std::string title)
+void endLogGroup(const std::string& title)
 {
     app()->endLogGroup(title) ;
 }
 
-void write_logfile(std::string file)
+void write_logfile(const std::string& file)
 {
     app()->write_logfile(file) ;
 }
@@ -127,14 +127,14 @@ void thread_complete()
     app()->thread_complete() ;
 }
 
-XML_Node* get_XML_File(std::string file, int debug)
+XML_Node* get_XML_File(const std::string& file, int debug)
 {
     XML_Node* xtmp = app()->get_XML_File(file, debug) ;
     //writelog("get_XML_File: returned from app:get_XML_FILE " + int2str(xtmp) + "\n");
     return xtmp;
 }
 
-void close_XML_File(std::string file)
+void close_XML_File(const std::string& file)
 {
     app()->close_XML_File(file) ;
 }
@@ -164,22 +164,22 @@ void showErrors()
     app()->logErrors() ;
 }
 
-void setError(std::string r, std::string msg)
+void setError(const std::string& r, const std::string& msg)
 {
     app()->addError(r, msg) ;
 }
 
-void addDirectory(std::string dir)
+void addDirectory(const std::string& dir)
 {
     app()->addDataDirectory(dir) ;
 }
 
-std::string findInputFile(std::string name)
+std::string findInputFile(const std::string& name)
 {
     return app()->findInputFile(name) ;
 }
 
-doublereal toSI(std::string unit)
+doublereal toSI(const std::string& unit)
 {
     doublereal f = Unit::units()->toSI(unit);
     if (f) {
@@ -190,7 +190,7 @@ doublereal toSI(std::string unit)
     return 1.0;
 }
 
-doublereal actEnergyToSI(std::string unit)
+doublereal actEnergyToSI(const std::string& unit)
 {
     doublereal f = Unit::units()->actEnergyToSI(unit);
     if (f) {
diff --git a/src/base/stringUtils.cpp b/src/base/stringUtils.cpp
index 493cf59de..da29d0cbd 100644
--- a/src/base/stringUtils.cpp
+++ b/src/base/stringUtils.cpp
@@ -299,17 +299,17 @@ std::string getBaseName(const std::string& path)
     return file;
 }
 //================================================================================================
-int intValue(std::string val)
+int intValue(const std::string& val)
 {
     return std::atoi(stripws(val).c_str());
 }
 //================================================================================================
-doublereal fpValue(std::string val)
+doublereal fpValue(const std::string& val)
 {
     return std::atof(stripws(val).c_str());
 }
 //================================================================================================
-doublereal fpValueCheck(std::string val)
+doublereal fpValueCheck(const std::string& val)
 {
     return atofCheck(stripws(val).c_str());
 }
diff --git a/src/base/units.h b/src/base/units.h
index 748a227d4..b93a30d36 100644
--- a/src/base/units.h
+++ b/src/base/units.h
@@ -58,7 +58,7 @@ public:
      * energy to SI units.
      * @param units activation energy units
      */
-    doublereal actEnergyToSI(std::string units) {
+    doublereal actEnergyToSI(const std::string& units) {
         if (m_act_u.find(units) != m_act_u.end()) {
             return m_act_u[units];
         } else {
@@ -75,7 +75,7 @@ public:
      *
      * @param units String containing the units description
      */
-    doublereal toSI(std::string units) {
+    doublereal toSI(const std::string& units) {
 
         // if dimensionless, return 1.0
         if (units == "") {
diff --git a/src/base/xml.cpp b/src/base/xml.cpp
index 417f923c0..a0c71803c 100644
--- a/src/base/xml.cpp
+++ b/src/base/xml.cpp
@@ -73,7 +73,7 @@ public:
      *  @param  closetag    String representing the closing of the XML bracket
      *  @param  line        Line number where the error occurred.
      */
-    XML_TagMismatch(std::string opentag, std::string closetag,
+    XML_TagMismatch(const std::string& opentag, const std::string& closetag,
                     int line=0) :
         XML_Error(line) {
         m_msg += "<" + opentag + "> paired with </" + closetag + ">.\n";
@@ -101,7 +101,7 @@ public:
      *  @param  child       Name of the required child node
      *  @param  line        Line number where the error occurred.
      */
-    XML_NoChild(const XML_Node* p, std::string parent,
+    XML_NoChild(const XML_Node* p, const std::string& parent,
                 std::string child, int line=0) :
         XML_Error(line) {
         m_msg += "           The XML Node \"" + parent +
@@ -147,7 +147,7 @@ void XML_Reader::getchr(char& ch)
  * @param q        Search for this character
  * @param istart   Defaults to 0
  */
-static string::size_type findUnbackslashed(std::string s, const char q,
+static string::size_type findUnbackslashed(const std::string& s, const char q,
         std::string::size_type istart = 0)
 {
     string::size_type iloc, icurrent, len;
@@ -220,7 +220,7 @@ int XML_Reader::findQuotedString(const std::string& s, std::string& rstring) con
  * parseTag parses XML tags, i.e., the XML elements that are
  * in between angle brackets.
  */
-void XML_Reader::parseTag(std::string tag, std::string& name,
+void XML_Reader::parseTag(const std::string& tag, std::string& name,
                           std::map<std::string, std::string>& attribs) const
 {
     string::size_type iloc;
@@ -690,7 +690,7 @@ std::string XML_Node::value(const std::string& cname) const
  *  @param cname  Name of the child node to the current
  *                node, for which you want the value
  */
-std::string XML_Node::operator()(std::string loc) const
+std::string XML_Node::operator()(const std::string& loc) const
 {
     return value(loc);
 }
@@ -823,7 +823,7 @@ XML_Node* XML_Node::setParent(XML_Node* const p)
  *
  * @return Returns true if the child node exists, false otherwise.
  */
-bool XML_Node::hasChild(const std::string ch) const
+bool XML_Node::hasChild(const std::string& ch) const
 {
     return (m_childindex.find(ch) != m_childindex.end());
 }
@@ -834,7 +834,7 @@ bool XML_Node::hasChild(const std::string ch) const
  *
  * @return Returns true if the attribute exists, false otherwise.
  */
-bool XML_Node::hasAttrib(std::string a) const
+bool XML_Node::hasAttrib(const std::string& a) const
 {
     return (m_attribs.find(a) != m_attribs.end());
 }
diff --git a/src/equil/MultiPhase.cpp b/src/equil/MultiPhase.cpp
index 537bbf552..75e7c62d0 100644
--- a/src/equil/MultiPhase.cpp
+++ b/src/equil/MultiPhase.cpp
@@ -321,7 +321,7 @@ doublereal MultiPhase::charge() const
     return sum;
 }
 //====================================================================================================================
-size_t MultiPhase::speciesIndex(std::string speciesName, std::string phaseName)
+size_t MultiPhase::speciesIndex(const std::string& speciesName, const std::string& phaseName)
 {
     if (!m_init) {
         init();
@@ -1035,7 +1035,7 @@ std::string MultiPhase::elementName(size_t m) const
 }
 //====================================================================================================================
 // Index of element with name \a name.
-size_t MultiPhase::elementIndex(std::string name) const
+size_t MultiPhase::elementIndex(const std::string& name) const
 {
     for (size_t e = 0; e < m_nel; e++) {
         if (m_enames[e] == name) {
diff --git a/src/equil/vcs_report.cpp b/src/equil/vcs_report.cpp
index 6410b2553..f23d6afd9 100644
--- a/src/equil/vcs_report.cpp
+++ b/src/equil/vcs_report.cpp
@@ -25,7 +25,7 @@ static void print_space(int num)
     }
 }
 
-static void print_line(std::string schar, size_t num)
+static void print_line(const std::string& schar, size_t num)
 {
     for (size_t j = 0; j < num; j++) {
         plogf("%s", schar.c_str());
diff --git a/src/kinetics/Kinetics.cpp b/src/kinetics/Kinetics.cpp
index 700be60f7..800d668a6 100644
--- a/src/kinetics/Kinetics.cpp
+++ b/src/kinetics/Kinetics.cpp
@@ -307,7 +307,7 @@ size_t Kinetics::kineticsSpeciesIndex(const std::string& nm,
  * phase where the species resides.
  * Will throw an error if the species string doesn't match.
  */
-thermo_t& Kinetics::speciesPhase(std::string nm)
+thermo_t& Kinetics::speciesPhase(const std::string& nm)
 {
     size_t np = m_thermo.size();
     size_t k;
@@ -407,7 +407,7 @@ void Kinetics::finalize()
 /*
  * @param m String message
  */
-void Kinetics::err(std::string m) const
+void Kinetics::err(const std::string& m) const
 {
     throw CanteraError("Kinetics::" + m,
                        "The default Base class method was called, when "
diff --git a/src/kinetics/KineticsFactory.cpp b/src/kinetics/KineticsFactory.cpp
index 4d2755364..f9f43c0c0 100644
--- a/src/kinetics/KineticsFactory.cpp
+++ b/src/kinetics/KineticsFactory.cpp
@@ -118,7 +118,7 @@ newKinetics(XML_Node& phaseData, vector<ThermoPhase*> th)
 /**
  * Return a new, empty kinetics manager.
  */
-Kinetics* KineticsFactory::newKinetics(string model)
+Kinetics* KineticsFactory::newKinetics(const string& model)
 {
 
     int ikin = -1;
diff --git a/src/kinetics/ReactionPath.cpp b/src/kinetics/ReactionPath.cpp
index e6a2ec9d9..7d9367cec 100644
--- a/src/kinetics/ReactionPath.cpp
+++ b/src/kinetics/ReactionPath.cpp
@@ -54,7 +54,7 @@ Path::Path(SpeciesNode* begin, SpeciesNode* end)
  * label.
  */
 void Path::addReaction(size_t rxnNumber, doublereal value,
-                       string label)
+                       const string& label)
 {
     m_rxn[rxnNumber] += value;
     m_total += value;
@@ -463,7 +463,7 @@ void ReactionPathDiagram::exportToDot(ostream& s)
 }
 
 
-void ReactionPathDiagram::addNode(size_t k, string nm, doublereal x)
+void ReactionPathDiagram::addNode(size_t k, const string& nm, doublereal x)
 {
     if (!m_nodes[k]) {
         m_nodes[k] = new SpeciesNode;
@@ -880,8 +880,8 @@ string reactionLabel(size_t i, size_t kr, size_t nr,
 }
 
 
-int ReactionPathBuilder::build(Kinetics& s,
-                               string element, ostream& output, ReactionPathDiagram& r, bool quiet)
+int ReactionPathBuilder::build(Kinetics& s, const string& element,
+                               ostream& output, ReactionPathDiagram& r, bool quiet)
 {
     doublereal f, ropf, ropr, fwd, rev;
     string fwdlabel, revlabel;
diff --git a/src/kinetics/ReactionStoichMgr.cpp b/src/kinetics/ReactionStoichMgr.cpp
index 0dcf815a1..205e1ef43 100644
--- a/src/kinetics/ReactionStoichMgr.cpp
+++ b/src/kinetics/ReactionStoichMgr.cpp
@@ -195,7 +195,7 @@ multiplyRevProducts(const doublereal* c, doublereal* r)
 
 
 void ReactionStoichMgr::
-write(string filename)
+write(const string& filename)
 {
     ofstream f(filename.c_str());
     f << "namespace mech {" << endl;
diff --git a/src/kinetics/importKinetics.cpp b/src/kinetics/importKinetics.cpp
index 2ec6056ed..2515e0438 100644
--- a/src/kinetics/importKinetics.cpp
+++ b/src/kinetics/importKinetics.cpp
@@ -1123,8 +1123,8 @@ bool importKinetics(const XML_Node& phase, std::vector<ThermoPhase*> th,
  * Build a single-phase ThermoPhase object with associated kinetics
  * mechanism.
  */
-bool buildSolutionFromXML(XML_Node& root, std::string id, std::string nm,
-                          ThermoPhase* th, Kinetics* k)
+bool buildSolutionFromXML(XML_Node& root, const std::string& id,
+                          const std::string& nm, ThermoPhase* th, Kinetics* k)
 {
     XML_Node* x;
     x = get_XML_NameID(nm, string("#")+id, &root);
diff --git a/src/numerics/BEulerInt.cpp b/src/numerics/BEulerInt.cpp
index e71642e29..80cb1aa75 100644
--- a/src/numerics/BEulerInt.cpp
+++ b/src/numerics/BEulerInt.cpp
@@ -35,7 +35,7 @@ namespace Cantera
  * Exception thrown when a BEuler error is encountered. We just call the
  * Cantera Error handler in the initialization list
  */
-BEulerErr::BEulerErr(std::string msg) :
+BEulerErr::BEulerErr(const std::string& msg) :
     CanteraError("BEulerInt", msg)
 {
 }
diff --git a/src/numerics/BandMatrix.cpp b/src/numerics/BandMatrix.cpp
index 1e2bd18ed..d2e06a310 100644
--- a/src/numerics/BandMatrix.cpp
+++ b/src/numerics/BandMatrix.cpp
@@ -340,7 +340,7 @@ ostream& operator<<(ostream& s, const BandMatrix& m)
     return s;
 }
 //====================================================================================================================
-void BandMatrix::err(std::string msg) const
+void BandMatrix::err(const std::string& msg) const
 {
     throw CanteraError("BandMatrix() unimplemented function", msg);
 }
diff --git a/src/numerics/CVodeInt.h b/src/numerics/CVodeInt.h
index 353ceb6b0..01c62cfda 100644
--- a/src/numerics/CVodeInt.h
+++ b/src/numerics/CVodeInt.h
@@ -20,7 +20,7 @@ namespace Cantera
 class CVodeErr : public CanteraError
 {
 public:
-    CVodeErr(std::string msg) : CanteraError("CVodeInt", msg) {}
+    explicit CVodeErr(const std::string& msg) : CanteraError("CVodeInt", msg) {}
 };
 
 
diff --git a/src/numerics/CVodesIntegrator.h b/src/numerics/CVodesIntegrator.h
index ca7fc67aa..68d6b3773 100644
--- a/src/numerics/CVodesIntegrator.h
+++ b/src/numerics/CVodesIntegrator.h
@@ -30,7 +30,7 @@ class FuncData;
 class CVodesErr : public CanteraError
 {
 public:
-    CVodesErr(std::string msg) : CanteraError("CVodesIntegrator", msg) {}
+    explicit CVodesErr(const std::string& msg) : CanteraError("CVodesIntegrator", msg) {}
 };
 
 
diff --git a/src/numerics/DAE_solvers.cpp b/src/numerics/DAE_solvers.cpp
index 9432d87d7..96fedbe08 100644
--- a/src/numerics/DAE_solvers.cpp
+++ b/src/numerics/DAE_solvers.cpp
@@ -20,7 +20,7 @@
 
 namespace Cantera
 {
-DAE_Solver* newDAE_Solver(std::string itype, ResidJacEval& f)
+DAE_Solver* newDAE_Solver(const std::string& itype, ResidJacEval& f)
 {
     if (itype == "IDA") {
 #ifdef HAS_SUNDIALS
diff --git a/src/numerics/Func1.cpp b/src/numerics/Func1.cpp
index fe2cfe923..a135f6263 100644
--- a/src/numerics/Func1.cpp
+++ b/src/numerics/Func1.cpp
@@ -148,7 +148,7 @@ void Func1::setParent(Func1* p)
 
 /*****************************************************************************/
 
-string Sin1::write(string arg) const
+string Sin1::write(const string& arg) const
 {
     string c  = "";
     if (m_c != 1.0) {
@@ -172,7 +172,7 @@ Func1& Cos1::derivative() const
     return *r;
 }
 
-std::string Cos1::write(std::string arg) const
+std::string Cos1::write(const std::string& arg) const
 {
     string c  = "";
     if (m_c != 1.0) {
@@ -193,7 +193,7 @@ Func1& Exp1::derivative() const
     }
 }
 
-std::string Exp1::write(std::string arg) const
+std::string Exp1::write(const std::string& arg) const
 {
     string c  = "";
     if (m_c != 1.0) {
@@ -218,7 +218,7 @@ Func1& Pow1::derivative() const
     return *r;
 }
 
-string Func1::write(std::string arg) const
+string Func1::write(const std::string& arg) const
 {
     return "<unknown " + int2str(ID()) + ">("+arg+")";
 }
@@ -226,7 +226,7 @@ string Func1::write(std::string arg) const
 
 
 
-string Pow1::write(string arg) const
+string Pow1::write(const std::string& arg) const
 {
     //cout << "Pow1" << endl;
     string c  = "";
@@ -245,7 +245,7 @@ string Pow1::write(string arg) const
 }
 
 
-string Const1::write(string arg) const
+string Const1::write(const std::string& arg) const
 {
     //cout << "Const1" << endl;
     string c  = "";
@@ -253,14 +253,14 @@ string Const1::write(string arg) const
     return c;
 }
 
-string Ratio1::write(string arg) const
+string Ratio1::write(const std::string& arg) const
 {
     //cout << "Ratio1" << endl;
     return "\\frac{" + m_f1->write(arg) + "}{"
            + m_f2->write(arg) + "}";
 }
 
-string Product1::write(string arg) const
+string Product1::write(const std::string& arg) const
 {
     //cout << "Product1" << endl;
     string s = m_f1->write(arg);
@@ -274,7 +274,7 @@ string Product1::write(string arg) const
     return s + " " + s2;
 }
 
-string Sum1::write(string arg) const
+string Sum1::write(const std::string& arg) const
 {
     //cout << "Sum1" << endl;
     string s1 = m_f1->write(arg);
@@ -286,7 +286,7 @@ string Sum1::write(string arg) const
     }
 }
 
-string Diff1::write(string arg) const
+string Diff1::write(const std::string& arg) const
 {
     //cout << "Diff1" << endl;
     string s1 = m_f1->write(arg);
@@ -298,14 +298,14 @@ string Diff1::write(string arg) const
     }
 }
 
-string Composite1::write(string arg) const
+string Composite1::write(const std::string& arg) const
 {
     //cout << "Composite1" << endl;
     string g = m_f2->write(arg);
     return m_f1->write(g);
 }
 
-string TimesConstant1::write(string arg) const
+string TimesConstant1::write(const std::string& arg) const
 {
     //cout << "TimesConstant1" << endl;
     string s = m_f1->write(arg);
@@ -325,7 +325,7 @@ string TimesConstant1::write(string arg) const
     return fp2str(m_c) + s;
 }
 
-string PlusConstant1::write(string arg) const
+string PlusConstant1::write(const std::string& arg) const
 {
     //cout << "PlusConstant1" << endl;
     if (m_c == 0.0) {
diff --git a/src/numerics/ODE_integrators.cpp b/src/numerics/ODE_integrators.cpp
index 252da5e45..03f68ff0e 100644
--- a/src/numerics/ODE_integrators.cpp
+++ b/src/numerics/ODE_integrators.cpp
@@ -11,7 +11,7 @@
 namespace Cantera
 {
 
-Integrator* newIntegrator(std::string itype)
+Integrator* newIntegrator(const std::string& itype)
 {
     if (itype == "CVODE") {
 #ifdef HAS_SUNDIALS
diff --git a/src/oneD/OneDim.cpp b/src/oneD/OneDim.cpp
index 3c034bfda..d4417d385 100644
--- a/src/oneD/OneDim.cpp
+++ b/src/oneD/OneDim.cpp
@@ -57,7 +57,7 @@ OneDim::OneDim(vector<Domain1D*> domains) :
 }
 
 
-size_t OneDim::domainIndex(string name)
+size_t OneDim::domainIndex(const std::string& name)
 {
     for (size_t n = 0; n < m_nd; n++) {
         if (domain(n).id() == name) {
@@ -473,7 +473,8 @@ doublereal OneDim::timeStep(int nsteps, doublereal dt, doublereal* x,
 }
 
 
-void OneDim::save(string fname, string id, string desc, doublereal* sol)
+void OneDim::save(const std::string& fname, std::string id,
+                  const std::string& desc, doublereal* sol)
 {
 
     struct tm* newtime;
diff --git a/src/oneD/Sim1D.cpp b/src/oneD/Sim1D.cpp
index 34a0b3f44..fced45404 100644
--- a/src/oneD/Sim1D.cpp
+++ b/src/oneD/Sim1D.cpp
@@ -53,7 +53,7 @@ Sim1D::Sim1D(vector<Domain1D*>& domains) :
 }
 //====================================================================================================================
 
-void Sim1D::setInitialGuess(string component, vector_fp& locs, vector_fp& vals)
+void Sim1D::setInitialGuess(const std::string& component, vector_fp& locs, vector_fp& vals)
 {
 
     for (size_t dom=0; dom<m_nd; dom++) {
@@ -143,7 +143,8 @@ void Sim1D::setProfile(size_t dom, size_t comp,
 }
 
 
-void Sim1D::save(string fname, string id, string desc)
+void Sim1D::save(const std::string& fname, const std::string& id,
+                 const std::string& desc)
 {
     OneDim::save(fname, id, desc, DATA_PTR(m_x));
 }
@@ -151,7 +152,7 @@ void Sim1D::save(string fname, string id, string desc)
 /**
  * Initialize the solution with a previously-saved solution.
  */
-void Sim1D::restore(string fname, string id)
+void Sim1D::restore(const std::string& fname, const std::string& id)
 {
     ifstream s(fname.c_str());
     //char buf[100];
diff --git a/src/oneD/StFlow.cpp b/src/oneD/StFlow.cpp
index 85fa1ae0c..9b67c36d1 100644
--- a/src/oneD/StFlow.cpp
+++ b/src/oneD/StFlow.cpp
@@ -1018,7 +1018,7 @@ string StFlow::componentName(size_t n) const
 }
 
 
-size_t StFlow::componentIndex(string name) const
+size_t StFlow::componentIndex(const std::string& name) const
 {
 
 
diff --git a/src/oneD/boundaries1D.cpp b/src/oneD/boundaries1D.cpp
index f444967d3..5109894c8 100644
--- a/src/oneD/boundaries1D.cpp
+++ b/src/oneD/boundaries1D.cpp
@@ -84,7 +84,7 @@ _init(size_t n)
 
 
 void Inlet1D::
-setMoleFractions(string xin)
+setMoleFractions(const std::string& xin)
 {
     m_xstr = xin;
     if (m_flow) {
@@ -537,7 +537,7 @@ restore(const XML_Node& dom, doublereal* soln)
 
 
 void OutletRes1D::
-setMoleFractions(string xres)
+setMoleFractions(const std::string& xres)
 {
     m_xstr = xres;
     if (m_flow) {
diff --git a/src/spectra/Nuclei.h b/src/spectra/Nuclei.h
index f8678d44f..866a1bab7 100644
--- a/src/spectra/Nuclei.h
+++ b/src/spectra/Nuclei.h
@@ -19,7 +19,7 @@ namespace Cantera
 class Nucleus
 {
 public:
-    Nucleus(std::string symbol,
+    Nucleus(const std::string& symbol,
             int nP, int nN, doublereal spin) : m_np(nP),
         m_nn(nN), m_spin(spin),
         m_sym(symbol) {}
diff --git a/src/thermo/DebyeHuckel.cpp b/src/thermo/DebyeHuckel.cpp
index 59b677737..fdcdadf84 100644
--- a/src/thermo/DebyeHuckel.cpp
+++ b/src/thermo/DebyeHuckel.cpp
@@ -63,7 +63,8 @@ DebyeHuckel::DebyeHuckel() :
  *  the routine initThermo(), with a reference to the
  *  XML database to get the info for the phase.
  */
-DebyeHuckel::DebyeHuckel(std::string inputFile, std::string id) :
+DebyeHuckel::DebyeHuckel(const std::string& inputFile,
+                         const std::string& id) :
     MolalityVPSSTP(),
     m_formDH(DHFORM_DILUTE_LIMIT),
     m_formGC(2),
@@ -85,7 +86,7 @@ DebyeHuckel::DebyeHuckel(std::string inputFile, std::string id) :
     initThermoFile(inputFile, id);
 }
 
-DebyeHuckel::DebyeHuckel(XML_Node& phaseRoot, std::string id) :
+DebyeHuckel::DebyeHuckel(XML_Node& phaseRoot, const std::string& id) :
     MolalityVPSSTP(),
     m_formDH(DHFORM_DILUTE_LIMIT),
     m_formGC(2),
@@ -910,7 +911,7 @@ void DebyeHuckel::initThermo()
 /*!
  *  @param estString  input string that will be interpreted
  */
-static int interp_est(std::string estString)
+static int interp_est(const std::string& estString)
 {
     const char* cc = estString.c_str();
     string lc = lowercase(estString);
@@ -954,7 +955,7 @@ static int interp_est(std::string estString)
  *             with the correct id.
  */
 void DebyeHuckel::
-initThermoXML(XML_Node& phaseNode, std::string id)
+initThermoXML(XML_Node& phaseNode, const std::string& id)
 {
     if (id.size() > 0) {
         std::string idp = phaseNode.id();
@@ -1668,7 +1669,7 @@ double DebyeHuckel::AionicRadius(int k) const
  * Bail out of functions with an error exit if they are not
  * implemented.
  */
-doublereal DebyeHuckel::err(std::string msg) const
+doublereal DebyeHuckel::err(const std::string& msg) const
 {
     throw CanteraError("DebyeHuckel",
                        "Unfinished func called: " + msg);
diff --git a/src/thermo/Elements.cpp b/src/thermo/Elements.cpp
index c0ece8d5c..674fa8f74 100644
--- a/src/thermo/Elements.cpp
+++ b/src/thermo/Elements.cpp
@@ -283,7 +283,7 @@ void Elements::freezeElements()
  * returned.
  *
  */
-int Elements::elementIndex(std::string name) const
+int Elements::elementIndex(const std::string& name) const
 {
     for (int i = 0; i < m_mm; i++) {
         if (m_elementNames[i] == name) {
diff --git a/src/thermo/FixedChemPotSSTP.cpp b/src/thermo/FixedChemPotSSTP.cpp
index 6503989a4..540ab708e 100644
--- a/src/thermo/FixedChemPotSSTP.cpp
+++ b/src/thermo/FixedChemPotSSTP.cpp
@@ -44,7 +44,7 @@ FixedChemPotSSTP::FixedChemPotSSTP() :
  * @param id     name of the phase id in the file.
  *               If this is blank, the first phase in the file is used.
  */
-FixedChemPotSSTP::FixedChemPotSSTP(std::string infile, std::string id) :
+FixedChemPotSSTP::FixedChemPotSSTP(const std::string& infile, std::string id) :
     SingleSpeciesTP(),
     chemPot_(0.0)
 {
@@ -72,7 +72,7 @@ FixedChemPotSSTP::FixedChemPotSSTP(std::string infile, std::string id) :
  *  @param phaseRef XML node pointing to a FixedChemPotSSTP description
  *  @param id       Id of the phase.
  */
-FixedChemPotSSTP::FixedChemPotSSTP(XML_Node& xmlphase, std::string id) :
+FixedChemPotSSTP::FixedChemPotSSTP(XML_Node& xmlphase, const std::string& id) :
     SingleSpeciesTP(),
     chemPot_(0.0)
 {
@@ -97,7 +97,7 @@ FixedChemPotSSTP::FixedChemPotSSTP(XML_Node& xmlphase, std::string id) :
     }
 }
 //====================================================================================================================
-FixedChemPotSSTP::FixedChemPotSSTP(std::string Ename, doublereal val) :
+FixedChemPotSSTP::FixedChemPotSSTP(const std::string& Ename, doublereal val) :
     SingleSpeciesTP(),
     chemPot_(0.0)
 {
@@ -499,7 +499,7 @@ void FixedChemPotSSTP::initThermo()
 }
 //====================================================================================================================
 
-void FixedChemPotSSTP::initThermoXML(XML_Node& phaseNode, std::string id)
+void FixedChemPotSSTP::initThermoXML(XML_Node& phaseNode, const std::string& id)
 {
     /*
      * Find the Thermo XML node
diff --git a/src/thermo/GeneralSpeciesThermo.cpp b/src/thermo/GeneralSpeciesThermo.cpp
index 7757acfdf..1805f1712 100644
--- a/src/thermo/GeneralSpeciesThermo.cpp
+++ b/src/thermo/GeneralSpeciesThermo.cpp
@@ -104,7 +104,7 @@ GeneralSpeciesThermo::duplMyselfAsSpeciesThermo() const
  * @param c        coefficients. The meaning of these depends on
  *                 the parameterization.
  */
-void GeneralSpeciesThermo::install(std::string name,
+void GeneralSpeciesThermo::install(const std::string& name,
                                    size_t index,
                                    int type,
                                    const doublereal* c,
diff --git a/src/thermo/GibbsExcessVPSSTP.cpp b/src/thermo/GibbsExcessVPSSTP.cpp
index fb2eced4f..5e019d565 100644
--- a/src/thermo/GibbsExcessVPSSTP.cpp
+++ b/src/thermo/GibbsExcessVPSSTP.cpp
@@ -299,7 +299,7 @@ void GibbsExcessVPSSTP::getPartialMolarVolumes(doublereal* vbar) const
 
 
 
-doublereal GibbsExcessVPSSTP::err(std::string msg) const
+doublereal GibbsExcessVPSSTP::err(const std::string& msg) const
 {
     throw CanteraError("GibbsExcessVPSSTP","Base class method "
                        +msg+" called. Equation of state type: "+int2str(eosType()));
diff --git a/src/thermo/HMWSoln.cpp b/src/thermo/HMWSoln.cpp
index ea1cb986f..3898099e7 100644
--- a/src/thermo/HMWSoln.cpp
+++ b/src/thermo/HMWSoln.cpp
@@ -91,7 +91,7 @@ HMWSoln::HMWSoln() :
  *  the routine initThermo(), with a reference to the
  *  XML database to get the info for the phase.
  */
-HMWSoln::HMWSoln(std::string inputFile, std::string id) :
+HMWSoln::HMWSoln(const std::string& inputFile, const std::string& id) :
     MolalityVPSSTP(),
     m_formPitzer(PITZERFORM_BASE),
     m_formPitzerTemp(PITZER_TEMP_CONSTANT),
@@ -142,7 +142,7 @@ HMWSoln::HMWSoln(std::string inputFile, std::string id) :
     initThermoFile(inputFile, id);
 }
 
-HMWSoln::HMWSoln(XML_Node& phaseRoot, std::string id) :
+HMWSoln::HMWSoln(XML_Node& phaseRoot, const std::string& id) :
     MolalityVPSSTP(),
     m_formPitzer(PITZERFORM_BASE),
     m_formPitzerTemp(PITZER_TEMP_CONSTANT),
@@ -1672,7 +1672,7 @@ double HMWSoln::AionicRadius(int k) const
  * Bail out of functions with an error exit if they are not
  * implemented.
  */
-doublereal HMWSoln::err(std::string msg) const
+doublereal HMWSoln::err(const std::string& msg) const
 {
     throw CanteraError("HMWSoln",
                        "Unfinished func called: " + msg);
diff --git a/src/thermo/HMWSoln_input.cpp b/src/thermo/HMWSoln_input.cpp
index 23c86f733..8c971690f 100644
--- a/src/thermo/HMWSoln_input.cpp
+++ b/src/thermo/HMWSoln_input.cpp
@@ -35,7 +35,7 @@ namespace Cantera
 /*!
  *  @param estString string name of the electrolyte species type
  */
-int HMWSoln::interp_est(std::string estString)
+int HMWSoln::interp_est(const std::string& estString)
 {
     const char* cc = estString.c_str();
     string lcs = lowercase(estString);
@@ -1066,7 +1066,7 @@ void HMWSoln::initThermo()
  *             with the correct id.
  */
 void HMWSoln::
-initThermoXML(XML_Node& phaseNode, std::string id)
+initThermoXML(XML_Node& phaseNode, const std::string& id)
 {
     string stemp;
     if (id.size() > 0) {
diff --git a/src/thermo/IdealMolalSoln.cpp b/src/thermo/IdealMolalSoln.cpp
index bea57e163..61507bbc2 100644
--- a/src/thermo/IdealMolalSoln.cpp
+++ b/src/thermo/IdealMolalSoln.cpp
@@ -106,7 +106,8 @@ operator=(const IdealMolalSoln& b)
     return *this;
 }
 
-IdealMolalSoln::IdealMolalSoln(std::string inputFile, std::string id) :
+IdealMolalSoln::IdealMolalSoln(const std::string& inputFile,
+                               const std::string& id) :
     MolalityVPSSTP(),
     m_formGC(2),
     IMS_typeCutoff_(0),
@@ -128,7 +129,7 @@ IdealMolalSoln::IdealMolalSoln(std::string inputFile, std::string id) :
     initThermoFile(inputFile, id);
 }
 
-IdealMolalSoln::IdealMolalSoln(XML_Node& root, std::string id) :
+IdealMolalSoln::IdealMolalSoln(XML_Node& root, const std::string& id) :
     MolalityVPSSTP(),
     m_formGC(2),
     IMS_typeCutoff_(0),
@@ -842,7 +843,7 @@ void IdealMolalSoln::initThermo()
  *             to see if phaseNode is pointing to the phase
  *             with the correct id.
  */
-void IdealMolalSoln::initThermoXML(XML_Node& phaseNode, std::string id)
+void IdealMolalSoln::initThermoXML(XML_Node& phaseNode, const std::string& id)
 {
     /*
      * Find the Thermo XML node
@@ -1060,7 +1061,7 @@ void IdealMolalSoln::setParametersFromXML(const XML_Node& eosdata)
  * Bail out of functions with an error exit if they are not
  * implemented.
  */
-doublereal IdealMolalSoln::err(std::string msg) const
+doublereal IdealMolalSoln::err(const std::string& msg) const
 {
     throw CanteraError("IdealMolalSoln",
                        "Unfinished func called: " + msg);
diff --git a/src/thermo/IdealSolidSolnPhase.cpp b/src/thermo/IdealSolidSolnPhase.cpp
index a6c652ddc..da4443e58 100644
--- a/src/thermo/IdealSolidSolnPhase.cpp
+++ b/src/thermo/IdealSolidSolnPhase.cpp
@@ -42,8 +42,9 @@ IdealSolidSolnPhase::IdealSolidSolnPhase(int formGC) :
     }
 }
 
-IdealSolidSolnPhase::IdealSolidSolnPhase(std::string inputFile, std::string id,
-        int formGC) :
+IdealSolidSolnPhase::IdealSolidSolnPhase(const std::string& inputFile,
+                                         const std::string& id,
+                                         int formGC) :
     ThermoPhase(),
     m_formGC(formGC),
     m_mm(0),
@@ -60,7 +61,7 @@ IdealSolidSolnPhase::IdealSolidSolnPhase(std::string inputFile, std::string id,
     initThermoFile(inputFile, id);
 }
 //====================================================================================================================
-IdealSolidSolnPhase::IdealSolidSolnPhase(XML_Node& root, std::string id,
+IdealSolidSolnPhase::IdealSolidSolnPhase(XML_Node& root, const std::string& id,
         int formGC) :
     ThermoPhase(),
     m_formGC(formGC),
@@ -1099,7 +1100,7 @@ void IdealSolidSolnPhase::initThermo()
  *             to see if phaseNode is pointing to the phase
  *             with the correct id.
  */
-void IdealSolidSolnPhase::initThermoXML(XML_Node& phaseNode, std::string id)
+void IdealSolidSolnPhase::initThermoXML(XML_Node& phaseNode, const std::string& id)
 {
     string subname = "IdealSolidSolnPhase::initThermoXML";
     if (id.size() > 0) {
diff --git a/src/thermo/IdealSolnGasVPSS.cpp b/src/thermo/IdealSolnGasVPSS.cpp
index 1004e4213..4fa8b5bdb 100644
--- a/src/thermo/IdealSolnGasVPSS.cpp
+++ b/src/thermo/IdealSolnGasVPSS.cpp
@@ -35,7 +35,7 @@ IdealSolnGasVPSS::IdealSolnGasVPSS() :
 }
 
 
-IdealSolnGasVPSS::IdealSolnGasVPSS(std::string infile, std::string id) :
+IdealSolnGasVPSS::IdealSolnGasVPSS(const std::string& infile, std::string id) :
     VPStandardStateTP(),
     m_idealGas(0),
     m_formGC(0)
@@ -489,7 +489,7 @@ void IdealSolnGasVPSS::initLengths()
  * This routine initializes the lengths in the current object and
  * then calls the parent routine.
  */
-void IdealSolnGasVPSS::initThermoXML(XML_Node& phaseNode, std::string id)
+void IdealSolnGasVPSS::initThermoXML(XML_Node& phaseNode, const std::string& id)
 {
     IdealSolnGasVPSS::initLengths();
 
diff --git a/src/thermo/IonsFromNeutralVPSSTP.cpp b/src/thermo/IonsFromNeutralVPSSTP.cpp
index d984341d8..70d23c8ef 100644
--- a/src/thermo/IonsFromNeutralVPSSTP.cpp
+++ b/src/thermo/IonsFromNeutralVPSSTP.cpp
@@ -80,7 +80,8 @@ IonsFromNeutralVPSSTP::IonsFromNeutralVPSSTP() :
  *                       If this parameter is zero, then a slave
  *                       neutral phase object is created and used.
  */
-IonsFromNeutralVPSSTP::IonsFromNeutralVPSSTP(std::string inputFile, std::string id,
+IonsFromNeutralVPSSTP::IonsFromNeutralVPSSTP(const std::string& inputFile,
+        const std::string& id,
         ThermoPhase* neutralPhase) :
     GibbsExcessVPSSTP(),
     ionSolnType_(cIonSolnType_SINGLEANION),
@@ -102,8 +103,8 @@ IonsFromNeutralVPSSTP::IonsFromNeutralVPSSTP(std::string inputFile, std::string
     initThermoFile(inputFile, id);
 }
 //====================================================================================================================
-IonsFromNeutralVPSSTP::IonsFromNeutralVPSSTP(XML_Node& phaseRoot, std::string id,
-        ThermoPhase* neutralPhase) :
+IonsFromNeutralVPSSTP::IonsFromNeutralVPSSTP(XML_Node& phaseRoot, 
+        const std::string& id, ThermoPhase* neutralPhase) :
     GibbsExcessVPSSTP(),
     ionSolnType_(cIonSolnType_SINGLEANION),
     numNeutralMoleculeSpecies_(0),
@@ -956,7 +957,7 @@ void IonsFromNeutralVPSSTP::setConcentrations(const doublereal* const c)
  */
 
 
-doublereal IonsFromNeutralVPSSTP::err(std::string msg) const
+doublereal IonsFromNeutralVPSSTP::err(const std::string& msg) const
 {
     throw CanteraError("IonsFromNeutralVPSSTP","Base class method "
                        +msg+" called. Equation of state type: "+int2str(eosType()));
@@ -1057,7 +1058,7 @@ static double factorOverlap(const std::vector<std::string>&  elnamesVN ,
  *             to see if phaseNode is pointing to the phase
  *             with the correct id.
  */
-void IonsFromNeutralVPSSTP::initThermoXML(XML_Node& phaseNode, std::string id)
+void IonsFromNeutralVPSSTP::initThermoXML(XML_Node& phaseNode, const std::string& id)
 {
     string stemp;
     if (id.size() > 0) {
diff --git a/src/thermo/LatticePhase.cpp b/src/thermo/LatticePhase.cpp
index 1ea20ae4f..11dcb0f21 100644
--- a/src/thermo/LatticePhase.cpp
+++ b/src/thermo/LatticePhase.cpp
@@ -87,7 +87,7 @@ LatticePhase::~LatticePhase()
  * @param inputFile String name of the input file
  * @param id        string id of the phase name
  */
-LatticePhase::LatticePhase(std::string inputFile, std::string id)
+LatticePhase::LatticePhase(const std::string& inputFile, const std::string& id)
 {
     initThermoFile(inputFile, id);
 }
@@ -97,7 +97,7 @@ LatticePhase::LatticePhase(std::string inputFile, std::string id)
  * @param phaseRef  XML node referencing the lattice phase.
  * @param id        string id of the phase name
  */
-LatticePhase::LatticePhase(XML_Node& phaseRef, std::string id)
+LatticePhase::LatticePhase(XML_Node& phaseRef, const std::string& id)
 {
     importPhase(*findXMLPhase(&phaseRef, id), this);
 }
@@ -428,7 +428,7 @@ void LatticePhase::initThermo()
     ThermoPhase::initThermo();
 }
 //====================================================================================================================
-void LatticePhase::initThermoXML(XML_Node& phaseNode, std::string id)
+void LatticePhase::initThermoXML(XML_Node& phaseNode, const std::string& id)
 {
     std::string idattrib = phaseNode.id();
     if (!id.empty() && id != idattrib) {
diff --git a/src/thermo/LatticeSolidPhase.cpp b/src/thermo/LatticeSolidPhase.cpp
index 30557d036..5ab58b6ee 100644
--- a/src/thermo/LatticeSolidPhase.cpp
+++ b/src/thermo/LatticeSolidPhase.cpp
@@ -608,7 +608,7 @@ void LatticeSolidPhase::_updateThermo() const
     }
 }
 //====================================================================================================================
-void LatticeSolidPhase::setLatticeMoleFractionsByName(int nn, std::string x)
+void LatticeSolidPhase::setLatticeMoleFractionsByName(int nn, const std::string& x)
 {
     m_lattice[nn]->setMoleFractionsByName(x);
     size_t loc = 0;
@@ -694,7 +694,7 @@ void LatticeSolidPhase::modifyOneHf298SS(const int k, const doublereal Hf298New)
 #endif
 //====================================================================================================================
 
-doublereal LatticeSolidPhase::err(std::string msg) const
+doublereal LatticeSolidPhase::err(const std::string& msg) const
 {
     throw CanteraError("LatticeSolidPhase","Unimplemented " + msg);
     return 0.0;
diff --git a/src/thermo/MargulesVPSSTP.cpp b/src/thermo/MargulesVPSSTP.cpp
index d2aa3966f..c8bac5abd 100644
--- a/src/thermo/MargulesVPSSTP.cpp
+++ b/src/thermo/MargulesVPSSTP.cpp
@@ -45,7 +45,7 @@ MargulesVPSSTP::MargulesVPSSTP() :
  *  XML database to get the info for the phase.
 
  */
-MargulesVPSSTP::MargulesVPSSTP(std::string inputFile, std::string id) :
+MargulesVPSSTP::MargulesVPSSTP(const std::string& inputFile, const std::string& id) :
     GibbsExcessVPSSTP(),
     numBinaryInteractions_(0),
     formMargules_(0),
@@ -54,7 +54,7 @@ MargulesVPSSTP::MargulesVPSSTP(std::string inputFile, std::string id) :
     initThermoFile(inputFile, id);
 }
 
-MargulesVPSSTP::MargulesVPSSTP(XML_Node& phaseRoot, std::string id) :
+MargulesVPSSTP::MargulesVPSSTP(XML_Node& phaseRoot, const std::string& id) :
     GibbsExcessVPSSTP(),
     numBinaryInteractions_(0),
     formMargules_(0),
@@ -502,7 +502,7 @@ void MargulesVPSSTP::getPartialMolarVolumes(doublereal* vbar) const
     }
 }
 
-doublereal MargulesVPSSTP::err(std::string msg) const
+doublereal MargulesVPSSTP::err(const std::string& msg) const
 {
     throw CanteraError("MargulesVPSSTP","Base class method "
                        +msg+" called. Equation of state type: "+int2str(eosType()));
@@ -553,7 +553,7 @@ void  MargulesVPSSTP::initLengths()
  *             to see if phaseNode is pointing to the phase
  *             with the correct id.
  */
-void MargulesVPSSTP::initThermoXML(XML_Node& phaseNode, std::string id)
+void MargulesVPSSTP::initThermoXML(XML_Node& phaseNode, const std::string& id)
 {
     string stemp;
     string subname = "MargulesVPSSTP::initThermoXML";
diff --git a/src/thermo/MetalSHEelectrons.cpp b/src/thermo/MetalSHEelectrons.cpp
index 3f526719a..cce3ace42 100644
--- a/src/thermo/MetalSHEelectrons.cpp
+++ b/src/thermo/MetalSHEelectrons.cpp
@@ -45,7 +45,7 @@ MetalSHEelectrons::MetalSHEelectrons():
  * @param id     name of the phase id in the file.
  *               If this is blank, the first phase in the file is used.
  */
-MetalSHEelectrons::MetalSHEelectrons(std::string infile, std::string id) :
+MetalSHEelectrons::MetalSHEelectrons(const std::string& infile, std::string id) :
     SingleSpeciesTP(),
     xdef_(0)
 {
@@ -79,7 +79,7 @@ MetalSHEelectrons::MetalSHEelectrons(std::string infile, std::string id) :
  *  @param phaseRef XML node pointing to a MetalSHEelectrons description
  *  @param id       Id of the phase.
  */
-MetalSHEelectrons::MetalSHEelectrons(XML_Node& xmlphase, std::string id) :
+MetalSHEelectrons::MetalSHEelectrons(XML_Node& xmlphase, const std::string& id) :
     SingleSpeciesTP(),
     xdef_(0)
 {
@@ -438,7 +438,7 @@ void MetalSHEelectrons::initThermo()
 }
 //====================================================================================================================
 
-void MetalSHEelectrons::initThermoXML(XML_Node& phaseNode, std::string id)
+void MetalSHEelectrons::initThermoXML(XML_Node& phaseNode, const std::string& id)
 {
     /*
      * Find the Thermo XML node
diff --git a/src/thermo/MineralEQ3.cpp b/src/thermo/MineralEQ3.cpp
index 09fad0a3e..74fa0b7b7 100644
--- a/src/thermo/MineralEQ3.cpp
+++ b/src/thermo/MineralEQ3.cpp
@@ -46,7 +46,7 @@ MineralEQ3::MineralEQ3():
  * @param id     name of the phase id in the file.
  *               If this is blank, the first phase in the file is used.
  */
-MineralEQ3::MineralEQ3(std::string infile, std::string id) :
+MineralEQ3::MineralEQ3(const std::string& infile, std::string id) :
     StoichSubstanceSSTP()
 {
     XML_Node* root = get_XML_File(infile);
@@ -73,7 +73,7 @@ MineralEQ3::MineralEQ3(std::string infile, std::string id) :
  *  @param phaseRef XML node pointing to a MineralEQ3 description
  *  @param id       Id of the phase.
  */
-MineralEQ3::MineralEQ3(XML_Node& xmlphase, std::string id) :
+MineralEQ3::MineralEQ3(XML_Node& xmlphase, const std::string& id) :
     StoichSubstanceSSTP()
 {
     if (id != "") {
@@ -479,7 +479,7 @@ void MineralEQ3::getParameters(int& n, doublereal* const c) const
  *             to see if phaseNode is pointing to the phase
  *             with the correct id.
  */
-void MineralEQ3::initThermoXML(XML_Node& phaseNode, std::string id)
+void MineralEQ3::initThermoXML(XML_Node& phaseNode, const std::string& id)
 {
     /*
      * Find the Thermo XML node
diff --git a/src/thermo/MixedSolventElectrolyte.cpp b/src/thermo/MixedSolventElectrolyte.cpp
index f7ffde22e..0d3cb9d2e 100644
--- a/src/thermo/MixedSolventElectrolyte.cpp
+++ b/src/thermo/MixedSolventElectrolyte.cpp
@@ -46,7 +46,8 @@ MixedSolventElectrolyte::MixedSolventElectrolyte() :
  *  XML database to get the info for the phase.
 
  */
-MixedSolventElectrolyte::MixedSolventElectrolyte(std::string inputFile, std::string id) :
+MixedSolventElectrolyte::MixedSolventElectrolyte(const std::string& inputFile,
+                                                 const std::string& id) :
     MolarityIonicVPSSTP(),
     numBinaryInteractions_(0),
     formMargules_(0),
@@ -55,7 +56,8 @@ MixedSolventElectrolyte::MixedSolventElectrolyte(std::string inputFile, std::str
     initThermoFile(inputFile, id);
 }
 
-MixedSolventElectrolyte::MixedSolventElectrolyte(XML_Node& phaseRoot, std::string id) :
+MixedSolventElectrolyte::MixedSolventElectrolyte(XML_Node& phaseRoot,
+                                                 const std::string& id) :
     MolarityIonicVPSSTP(),
     numBinaryInteractions_(0),
     formMargules_(0),
@@ -503,7 +505,7 @@ void MixedSolventElectrolyte::getPartialMolarVolumes(doublereal* vbar) const
     }
 }
 
-doublereal MixedSolventElectrolyte::err(std::string msg) const
+doublereal MixedSolventElectrolyte::err(const std::string& msg) const
 {
     throw CanteraError("MixedSolventElectrolyte","Base class method "
                        +msg+" called. Equation of state type: "+int2str(eosType()));
@@ -554,7 +556,7 @@ void  MixedSolventElectrolyte::initLengths()
  *             to see if phaseNode is pointing to the phase
  *             with the correct id.
  */
-void MixedSolventElectrolyte::initThermoXML(XML_Node& phaseNode, std::string id)
+void MixedSolventElectrolyte::initThermoXML(XML_Node& phaseNode, const std::string& id)
 {
     string subname = "MixedSolventElectrolyte::initThermoXML";
     string stemp;
diff --git a/src/thermo/MixtureFugacityTP.cpp b/src/thermo/MixtureFugacityTP.cpp
index e6b728d97..a589958bd 100644
--- a/src/thermo/MixtureFugacityTP.cpp
+++ b/src/thermo/MixtureFugacityTP.cpp
@@ -190,7 +190,7 @@ int  MixtureFugacityTP::reportSolnBranchActual() const
  */
 //====================================================================================================================
 
-doublereal MixtureFugacityTP::err(std::string msg) const
+doublereal MixtureFugacityTP::err(const std::string& msg) const
 {
     throw CanteraError("MixtureFugacityTP","Base class method "
                        +msg+" called. Equation of state type: "+int2str(eosType()));
@@ -707,7 +707,7 @@ void MixtureFugacityTP::setState_TPX(doublereal t, doublereal p, const doublerea
  * This routine initializes the lengths in the current object and
  * then calls the parent routine.
  */
-void MixtureFugacityTP::initThermoXML(XML_Node& phaseNode, std::string id)
+void MixtureFugacityTP::initThermoXML(XML_Node& phaseNode, const std::string& id)
 {
     MixtureFugacityTP::initLengths();
 
diff --git a/src/thermo/MolalityVPSSTP.cpp b/src/thermo/MolalityVPSSTP.cpp
index c7a0fd1a7..ead8b9cbd 100644
--- a/src/thermo/MolalityVPSSTP.cpp
+++ b/src/thermo/MolalityVPSSTP.cpp
@@ -559,7 +559,7 @@ void MolalityVPSSTP::getElectrochemPotentials(doublereal* mu) const
  */
 
 
-doublereal MolalityVPSSTP::err(std::string msg) const
+doublereal MolalityVPSSTP::err(const std::string& msg) const
 {
     throw CanteraError("MolalityVPSSTP","Base class method "
                        +msg+" called. Equation of state type: "+int2str(eosType()));
@@ -814,7 +814,7 @@ void  MolalityVPSSTP::initLengths()
  *             to see if phaseNode is pointing to the phase
  *             with the correct id.
  */
-void MolalityVPSSTP::initThermoXML(XML_Node& phaseNode, std::string id)
+void MolalityVPSSTP::initThermoXML(XML_Node& phaseNode, const std::string& id)
 {
 
     initLengths();
diff --git a/src/thermo/MolarityIonicVPSSTP.cpp b/src/thermo/MolarityIonicVPSSTP.cpp
index 253fd7f1c..e31221535 100644
--- a/src/thermo/MolarityIonicVPSSTP.cpp
+++ b/src/thermo/MolarityIonicVPSSTP.cpp
@@ -53,7 +53,8 @@ MolarityIonicVPSSTP::MolarityIonicVPSSTP() :
  *  the routine initThermo(), with a reference to the
  *  XML database to get the info for the phase.
  */
-MolarityIonicVPSSTP::MolarityIonicVPSSTP(std::string inputFile, std::string id) :
+MolarityIonicVPSSTP::MolarityIonicVPSSTP(const std::string& inputFile,
+                                         const std::string& id) :
     GibbsExcessVPSSTP(),
     PBType_(PBTYPE_PASSTHROUGH),
     numPBSpecies_(m_kk),
@@ -66,7 +67,8 @@ MolarityIonicVPSSTP::MolarityIonicVPSSTP(std::string inputFile, std::string id)
     initThermoFile(inputFile, id);
 }
 //====================================================================================================================
-MolarityIonicVPSSTP::MolarityIonicVPSSTP(XML_Node& phaseRoot, std::string id) :
+MolarityIonicVPSSTP::MolarityIonicVPSSTP(XML_Node& phaseRoot,
+                                         const std::string& id) :
     GibbsExcessVPSSTP(),
     PBType_(PBTYPE_PASSTHROUGH),
     numPBSpecies_(m_kk),
@@ -486,7 +488,7 @@ void  MolarityIonicVPSSTP::s_update_dlnActCoeff_dX_() const
  * ------------ Partial Molar Properties of the Solution ------------
  */
 //====================================================================================================================
-doublereal MolarityIonicVPSSTP::err(std::string msg) const
+doublereal MolarityIonicVPSSTP::err(const std::string& msg) const
 {
     throw CanteraError("MolarityIonicVPSSTP","Base class method "
                        +msg+" called. Equation of state type: "+int2str(eosType()));
@@ -566,7 +568,7 @@ void  MolarityIonicVPSSTP::initLengths()
  *             to see if phaseNode is pointing to the phase
  *             with the correct id.
  */
-void MolarityIonicVPSSTP::initThermoXML(XML_Node& phaseNode, std::string id)
+void MolarityIonicVPSSTP::initThermoXML(XML_Node& phaseNode, const std::string& id)
 {
     std::string subname = "MolarityIonicVPSSTP::initThermoXML";
     std::string stemp;
diff --git a/src/thermo/Mu0Poly.cpp b/src/thermo/Mu0Poly.cpp
index 56ac1d295..6ded6bf19 100644
--- a/src/thermo/Mu0Poly.cpp
+++ b/src/thermo/Mu0Poly.cpp
@@ -202,7 +202,7 @@ void Mu0Poly::modifyParameters(doublereal* coeffs)
  * parameterization for species k into a SpeciesThermo instance,
  * getting the information from an XML database.
  */
-void installMu0ThermoFromXML(std::string speciesName,
+void installMu0ThermoFromXML(const std::string& speciesName,
                              SpeciesThermo& sp, size_t k,
                              const XML_Node* Mu0Node_ptr)
 {
diff --git a/src/thermo/NasaThermo.h b/src/thermo/NasaThermo.h
index 7028c8443..c4f064e6d 100644
--- a/src/thermo/NasaThermo.h
+++ b/src/thermo/NasaThermo.h
@@ -150,7 +150,7 @@ public:
      *                    parameterization.
      * @see speciesThermoTypes.h
      */
-    virtual void install(std::string name, size_t index, int type,
+    virtual void install(const std::string& name, size_t index, int type,
                          const doublereal* c,
                          doublereal minTemp, doublereal maxTemp,
                          doublereal refPressure) {
@@ -558,8 +558,8 @@ private:
      * @param clow  coefficients for lower temperature region
      * @param chigh coefficients for higher temperature region
      */
-    void checkContinuity(std::string name, double tmid, const doublereal* clow,
-                         doublereal* chigh);
+    void checkContinuity(const std::string& name, double tmid,
+                         const doublereal* clow, doublereal* chigh);
 
     //! for internal use by checkContinuity
     /*!
diff --git a/src/thermo/PDSS.cpp b/src/thermo/PDSS.cpp
index 12ef83f48..c001449d7 100644
--- a/src/thermo/PDSS.cpp
+++ b/src/thermo/PDSS.cpp
@@ -196,7 +196,7 @@ PDSS_enumType PDSS::reportPDSSType() const
     return m_pdssType;
 }
 
-void PDSS::initThermoXML(const XML_Node& phaseNode, std::string& id)
+void PDSS::initThermoXML(const XML_Node& phaseNode, const std::string& id)
 {
     AssertThrow(m_tp != 0, "PDSS::initThermoXML()");
     m_p0 =  m_vpssmgr_ptr->refPressure(m_spindex);
@@ -525,7 +525,7 @@ doublereal PDSS::satPressure(doublereal t)
 }
 
 
-void PDSS::err(std::string msg) const
+void PDSS::err(const std::string& msg) const
 {
     throw CanteraError("PDSS::" + msg, "unimplemented");
 }
diff --git a/src/thermo/PDSS_ConstVol.cpp b/src/thermo/PDSS_ConstVol.cpp
index 97abb3f88..81d216176 100644
--- a/src/thermo/PDSS_ConstVol.cpp
+++ b/src/thermo/PDSS_ConstVol.cpp
@@ -33,7 +33,8 @@ PDSS_ConstVol::PDSS_ConstVol(VPStandardStateTP* tp, size_t spindex) :
 }
 
 
-PDSS_ConstVol::PDSS_ConstVol(VPStandardStateTP* tp, size_t spindex, std::string inputFile, std::string id) :
+PDSS_ConstVol::PDSS_ConstVol(VPStandardStateTP* tp, size_t spindex,
+                             const std::string& inputFile, const std::string& id) :
     PDSS(tp, spindex)
 {
     m_pdssType = cPDSS_CONSTVOL;
@@ -148,7 +149,8 @@ void PDSS_ConstVol::constructPDSSXML(VPStandardStateTP* tp, size_t spindex,
  *            phase element will be used.
  */
 void PDSS_ConstVol::constructPDSSFile(VPStandardStateTP* tp, size_t spindex,
-                                      std::string inputFile, std::string id)
+                                      const std::string& inputFile,
+                                      const std::string& id)
 {
 
     if (inputFile.size() == 0) {
diff --git a/src/thermo/PDSS_HKFT.cpp b/src/thermo/PDSS_HKFT.cpp
index 4afcff6f9..9f8dacf48 100644
--- a/src/thermo/PDSS_HKFT.cpp
+++ b/src/thermo/PDSS_HKFT.cpp
@@ -59,7 +59,8 @@ PDSS_HKFT::PDSS_HKFT(VPStandardStateTP* tp, size_t spindex) :
 }
 
 
-PDSS_HKFT::PDSS_HKFT(VPStandardStateTP* tp, size_t spindex, std::string inputFile, std::string id) :
+PDSS_HKFT::PDSS_HKFT(VPStandardStateTP* tp, size_t spindex,
+                     const std::string& inputFile, const std::string& id) :
     PDSS(tp, spindex),
     m_waterSS(0),
     m_densWaterSS(-1.0),
@@ -809,7 +810,8 @@ void PDSS_HKFT::constructPDSSXML(VPStandardStateTP* tp, size_t spindex,
 }
 
 void PDSS_HKFT::constructPDSSFile(VPStandardStateTP* tp, size_t spindex,
-                                  std::string inputFile, std::string id)
+                                  const std::string& inputFile,
+                                  const std::string& id)
 {
 
     if (inputFile.size() == 0) {
diff --git a/src/thermo/PDSS_IdealGas.cpp b/src/thermo/PDSS_IdealGas.cpp
index 562258f76..72f7f3613 100644
--- a/src/thermo/PDSS_IdealGas.cpp
+++ b/src/thermo/PDSS_IdealGas.cpp
@@ -34,7 +34,7 @@ PDSS_IdealGas::PDSS_IdealGas(VPStandardStateTP* tp, int spindex) :
 
 
 PDSS_IdealGas::PDSS_IdealGas(VPStandardStateTP* tp, int spindex,
-                             std::string inputFile, std::string id) :
+                             const std::string& inputFile, const std::string& id) :
     PDSS(tp, spindex)
 {
     m_pdssType = cPDSS_IDEALGAS;
@@ -113,7 +113,7 @@ PDSS* PDSS_IdealGas::duplMyselfAsPDSS() const
  *            phase element will be used.
  */
 void PDSS_IdealGas::constructPDSSXML(VPStandardStateTP* tp, size_t spindex,
-                                     const XML_Node& phaseNode, std::string id)
+                                     const XML_Node& phaseNode, const std::string& id)
 {
     //initThermo();
     //initThermoXML(phaseNode, id);
@@ -121,7 +121,8 @@ void PDSS_IdealGas::constructPDSSXML(VPStandardStateTP* tp, size_t spindex,
 
 
 void PDSS_IdealGas::constructPDSSFile(VPStandardStateTP* tp, size_t spindex,
-                                      std::string inputFile, std::string id)
+                                      const std::string& inputFile,
+                                      const std::string& id)
 {
 
     if (inputFile.size() == 0) {
diff --git a/src/thermo/PDSS_IonsFromNeutral.cpp b/src/thermo/PDSS_IonsFromNeutral.cpp
index a74b578d5..480777fca 100644
--- a/src/thermo/PDSS_IonsFromNeutral.cpp
+++ b/src/thermo/PDSS_IonsFromNeutral.cpp
@@ -37,7 +37,7 @@ PDSS_IonsFromNeutral::PDSS_IonsFromNeutral(VPStandardStateTP* tp, size_t spindex
 }
 //====================================================================================================================
 PDSS_IonsFromNeutral::PDSS_IonsFromNeutral(VPStandardStateTP* tp, size_t spindex,
-        std::string inputFile, std::string id) :
+                                           const std::string& inputFile, const std::string& id) :
     PDSS(tp, spindex),
     neutralMoleculePhase_(0),
     numMult_(0),
@@ -154,7 +154,7 @@ void PDSS_IonsFromNeutral::initAllPtrs(VPStandardStateTP* tp, VPSSMgr* vpssmgr_p
  */
 void PDSS_IonsFromNeutral::constructPDSSXML(VPStandardStateTP* tp, size_t spindex,
         const XML_Node& speciesNode,
-        const XML_Node& phaseNode, std::string id)
+        const XML_Node& phaseNode, const std::string& id)
 {
     const XML_Node* tn = speciesNode.findByName("thermo");
     if (!tn) {
@@ -227,7 +227,7 @@ void PDSS_IonsFromNeutral::constructPDSSXML(VPStandardStateTP* tp, size_t spinde
  *                    phase element will be used.
  */
 void PDSS_IonsFromNeutral::constructPDSSFile(VPStandardStateTP* tp, size_t spindex,
-        std::string inputFile, std::string id)
+                                             const std::string& inputFile, const std::string& id)
 {
 
     if (inputFile.size() == 0) {
diff --git a/src/thermo/PDSS_SSVol.cpp b/src/thermo/PDSS_SSVol.cpp
index 7dc33ec14..649a22f54 100644
--- a/src/thermo/PDSS_SSVol.cpp
+++ b/src/thermo/PDSS_SSVol.cpp
@@ -39,7 +39,7 @@ PDSS_SSVol::PDSS_SSVol(VPStandardStateTP* tp, size_t spindex) :
 
 
 PDSS_SSVol::PDSS_SSVol(VPStandardStateTP* tp,
-                       size_t spindex, std::string inputFile, std::string id) :
+                       size_t spindex, const std::string& inputFile, const std::string& id) :
     PDSS(tp, spindex),
     volumeModel_(cSSVOLUME_CONSTANT),
     m_constMolarVolume(-1.0)
@@ -177,7 +177,7 @@ void PDSS_SSVol::constructPDSSXML(VPStandardStateTP* tp, size_t spindex,
  *            phase element will be used.
  */
 void PDSS_SSVol::constructPDSSFile(VPStandardStateTP* tp, size_t spindex,
-                                   std::string inputFile, std::string id)
+                                   const std::string& inputFile, const std::string& id)
 {
 
     if (inputFile.size() == 0) {
diff --git a/src/thermo/PDSS_Water.cpp b/src/thermo/PDSS_Water.cpp
index ac3067b57..a00a20d07 100644
--- a/src/thermo/PDSS_Water.cpp
+++ b/src/thermo/PDSS_Water.cpp
@@ -69,7 +69,7 @@ PDSS_Water::PDSS_Water(VPStandardStateTP* tp, int spindex) :
 
 
 PDSS_Water::PDSS_Water(VPStandardStateTP* tp, int spindex,
-                       std::string inputFile, std::string id) :
+                       const std::string& inputFile, const std::string& id) :
     PDSS(tp, spindex),
     m_sub(0),
     m_waterProps(0),
@@ -196,7 +196,7 @@ PDSS* PDSS_Water::duplMyselfAsPDSS() const
  *            phase element will be used.
  */
 void PDSS_Water::constructPDSSXML(VPStandardStateTP* tp, int spindex,
-                                  const XML_Node& phaseNode, std::string id)
+                                  const XML_Node& phaseNode, const std::string& id)
 {
     constructSet();
 }
@@ -218,7 +218,7 @@ void PDSS_Water::constructPDSSXML(VPStandardStateTP* tp, int spindex,
  *            phase element will be used.
  */
 void PDSS_Water::constructPDSSFile(VPStandardStateTP* tp, int spindex,
-                                   std::string inputFile, std::string id)
+                                   const std::string& inputFile, const std::string& id)
 {
 
     if (inputFile.size() == 0) {
diff --git a/src/thermo/Phase.cpp b/src/thermo/Phase.cpp
index 7b215d29a..866f6d1d0 100644
--- a/src/thermo/Phase.cpp
+++ b/src/thermo/Phase.cpp
@@ -122,7 +122,7 @@ std::string Phase::id() const
     return m_id;
 }
 
-void Phase::setID(std::string id)
+void Phase::setID(const std::string& id)
 {
     m_id = id;
 }
@@ -132,7 +132,7 @@ std::string Phase::name() const
     return m_name;
 }
 
-void Phase::setName(std::string nm)
+void Phase::setName(const std::string& nm)
 {
     m_name = nm;
 }
@@ -162,7 +162,7 @@ string Phase::elementName(size_t m) const
     return m_elementNames[m];
 }
 
-size_t Phase::elementIndex(std::string name) const
+size_t Phase::elementIndex(const std::string& name) const
 {
     for (size_t i = 0; i < m_mm; i++) {
         if (m_elementNames[i] == name) {
@@ -227,7 +227,7 @@ void Phase::getAtoms(size_t k, double* atomArray) const
     }
 }
 
-size_t Phase::speciesIndex(std::string nameStr) const
+size_t Phase::speciesIndex(const std::string& nameStr) const
 {
     std::string pn;
     std::string sn = parseSpeciesName(nameStr, pn);
@@ -521,7 +521,7 @@ doublereal Phase::moleFraction(size_t k) const
     return m_ym[k] * m_mmw;
 }
 
-doublereal Phase::moleFraction(std::string nameSpec) const
+doublereal Phase::moleFraction(const std::string& nameSpec) const
 {
     size_t iloc = speciesIndex(nameSpec);
     if (iloc != npos) {
@@ -542,7 +542,7 @@ doublereal Phase::massFraction(size_t k) const
     return m_y[k];
 }
 
-doublereal Phase::massFraction(std::string nameSpec) const
+doublereal Phase::massFraction(const std::string& nameSpec) const
 {
     size_t iloc = speciesIndex(nameSpec);
     if (iloc != npos) {
diff --git a/src/thermo/PhaseCombo_Interaction.cpp b/src/thermo/PhaseCombo_Interaction.cpp
index caa309163..0ffe62783 100644
--- a/src/thermo/PhaseCombo_Interaction.cpp
+++ b/src/thermo/PhaseCombo_Interaction.cpp
@@ -45,7 +45,8 @@ PhaseCombo_Interaction::PhaseCombo_Interaction() :
  *
  * HKM - Checked for Transition
  */
-PhaseCombo_Interaction::PhaseCombo_Interaction(std::string inputFile, std::string id) :
+PhaseCombo_Interaction::PhaseCombo_Interaction(const std::string& inputFile,
+                                               const std::string& id) :
     GibbsExcessVPSSTP(),
     numBinaryInteractions_(0),
     formMargules_(0),
@@ -59,7 +60,8 @@ PhaseCombo_Interaction::PhaseCombo_Interaction(std::string inputFile, std::strin
  *
  * HKM - Checked for Transition
  */
-PhaseCombo_Interaction::PhaseCombo_Interaction(XML_Node& phaseRoot, std::string id) :
+PhaseCombo_Interaction::PhaseCombo_Interaction(XML_Node& phaseRoot,
+                                               const std::string& id) :
     GibbsExcessVPSSTP(),
     numBinaryInteractions_(0),
     formMargules_(0),
@@ -513,7 +515,7 @@ void PhaseCombo_Interaction::getPartialMolarVolumes(doublereal* vbar) const
     }
 }
 //====================================================================================================================
-doublereal PhaseCombo_Interaction::err(std::string msg) const
+doublereal PhaseCombo_Interaction::err(const std::string& msg) const
 {
     throw CanteraError("PhaseCombo_Interaction","Base class method "
                        +msg+" called. Equation of state type: "+int2str(eosType()));
@@ -564,7 +566,7 @@ void  PhaseCombo_Interaction::initLengths()
  *             to see if phaseNode is pointing to the phase
  *             with the correct id.
  */
-void PhaseCombo_Interaction::initThermoXML(XML_Node& phaseNode, std::string id)
+void PhaseCombo_Interaction::initThermoXML(XML_Node& phaseNode, const std::string& id)
 {
     string subname = "PhaseCombo_Interaction::initThermoXML";
     string stemp;
diff --git a/src/thermo/PseudoBinaryVPSSTP.cpp b/src/thermo/PseudoBinaryVPSSTP.cpp
index 1075de39e..7830eb416 100644
--- a/src/thermo/PseudoBinaryVPSSTP.cpp
+++ b/src/thermo/PseudoBinaryVPSSTP.cpp
@@ -237,7 +237,7 @@ void PseudoBinaryVPSSTP::calcPseudoBinaryMoleFractions() const
  */
 
 
-doublereal PseudoBinaryVPSSTP::err(std::string msg) const
+doublereal PseudoBinaryVPSSTP::err(const std::string& msg) const
 {
     throw CanteraError("PseudoBinaryVPSSTP","Base class method "
                        +msg+" called. Equation of state type: "+int2str(eosType()));
@@ -288,7 +288,7 @@ void  PseudoBinaryVPSSTP::initLengths()
  *             to see if phaseNode is pointing to the phase
  *             with the correct id.
  */
-void PseudoBinaryVPSSTP::initThermoXML(XML_Node& phaseNode, std::string id)
+void PseudoBinaryVPSSTP::initThermoXML(XML_Node& phaseNode, const std::string& id)
 {
 
 
diff --git a/src/thermo/RedlichKisterVPSSTP.cpp b/src/thermo/RedlichKisterVPSSTP.cpp
index 1da976bbc..c61fa6d89 100644
--- a/src/thermo/RedlichKisterVPSSTP.cpp
+++ b/src/thermo/RedlichKisterVPSSTP.cpp
@@ -53,7 +53,8 @@ RedlichKisterVPSSTP::RedlichKisterVPSSTP() :
  *  XML database to get the info for the phase.
 
  */
-RedlichKisterVPSSTP::RedlichKisterVPSSTP(std::string inputFile, std::string id) :
+RedlichKisterVPSSTP::RedlichKisterVPSSTP(const std::string& inputFile,
+                                         const std::string& id) :
     GibbsExcessVPSSTP(),
     numBinaryInteractions_(0),
     m_pSpecies_A_ij(0),
@@ -68,7 +69,8 @@ RedlichKisterVPSSTP::RedlichKisterVPSSTP(std::string inputFile, std::string id)
     initThermoFile(inputFile, id);
 }
 //====================================================================================================================
-RedlichKisterVPSSTP::RedlichKisterVPSSTP(XML_Node& phaseRoot, std::string id) :
+RedlichKisterVPSSTP::RedlichKisterVPSSTP(XML_Node& phaseRoot,
+                                         const std::string& id) :
     GibbsExcessVPSSTP(),
     numBinaryInteractions_(0),
     m_pSpecies_A_ij(0),
@@ -477,7 +479,7 @@ void RedlichKisterVPSSTP::getPartialMolarVolumes(doublereal* vbar) const
     }
 }
 //====================================================================================================================
-doublereal RedlichKisterVPSSTP::err(std::string msg) const
+doublereal RedlichKisterVPSSTP::err(const std::string& msg) const
 {
     throw CanteraError("RedlichKisterVPSSTP","Base class method "
                        +msg+" called. Equation of state type: "+int2str(eosType()));
@@ -522,7 +524,7 @@ void  RedlichKisterVPSSTP::initLengths()
  * @param id   ID of the phase. If nonnull, a check is done to see if phaseNode is pointing to the phase
  *             with the correct id.
  */
-void RedlichKisterVPSSTP::initThermoXML(XML_Node& phaseNode, std::string id)
+void RedlichKisterVPSSTP::initThermoXML(XML_Node& phaseNode, const std::string& id)
 {
     std::string subname = "RedlichKisterVPSSTP::initThermoXML";
     std::string stemp;
diff --git a/src/thermo/RedlichKwongMFTP.cpp b/src/thermo/RedlichKwongMFTP.cpp
index f83c8fa16..a0404f5be 100644
--- a/src/thermo/RedlichKwongMFTP.cpp
+++ b/src/thermo/RedlichKwongMFTP.cpp
@@ -57,7 +57,7 @@ RedlichKwongMFTP::RedlichKwongMFTP() :
     Vroot_[2] = 0.0;
 }
 //====================================================================================================================
-RedlichKwongMFTP::RedlichKwongMFTP(std::string infile, std::string id) :
+RedlichKwongMFTP::RedlichKwongMFTP(const std::string& infile, std::string id) :
     MixtureFugacityTP(),
     m_standardMixingRules(0),
     m_formTempParam(0),
@@ -92,7 +92,7 @@ RedlichKwongMFTP::RedlichKwongMFTP(std::string infile, std::string id) :
     importPhase(*xphase, this);
 }
 //====================================================================================================================
-RedlichKwongMFTP::RedlichKwongMFTP(XML_Node& phaseRefRoot, std::string id) :
+RedlichKwongMFTP::RedlichKwongMFTP(XML_Node& phaseRefRoot, const std::string& id) :
     MixtureFugacityTP(),
     m_standardMixingRules(0),
     m_formTempParam(0),
@@ -935,7 +935,7 @@ void RedlichKwongMFTP::initLengths()
  * This routine initializes the lengths in the current object and
  * then calls the parent routine.
  */
-void RedlichKwongMFTP::initThermoXML(XML_Node& phaseNode, std::string id)
+void RedlichKwongMFTP::initThermoXML(XML_Node& phaseNode, const std::string& id)
 {
     RedlichKwongMFTP::initLengths();
 
diff --git a/src/thermo/ShomateThermo.h b/src/thermo/ShomateThermo.h
index 7c808b217..e978d3ef1 100644
--- a/src/thermo/ShomateThermo.h
+++ b/src/thermo/ShomateThermo.h
@@ -163,7 +163,7 @@ public:
      * @see ShomatePoly
      * @see ShomatePoly2
      */
-    virtual void install(std::string name, size_t index, int type,
+    virtual void install(const std::string& name, size_t index, int type,
                          const doublereal* c,
                          doublereal minTemp, doublereal maxTemp,
                          doublereal refPressure) {
diff --git a/src/thermo/SingleSpeciesTP.cpp b/src/thermo/SingleSpeciesTP.cpp
index 9c0562c3c..fe7b469f3 100644
--- a/src/thermo/SingleSpeciesTP.cpp
+++ b/src/thermo/SingleSpeciesTP.cpp
@@ -623,7 +623,7 @@ void SingleSpeciesTP::setState_SV(doublereal s, doublereal v,
  * this class doesn't have an answer for the question given to it,
  *  because the derived class isn't overriding a function.
  */
-doublereal SingleSpeciesTP::err(std::string msg) const
+doublereal SingleSpeciesTP::err(const std::string& msg) const
 {
     throw CanteraError("SingleSpeciesTP","Base class method "
                        +msg+" called. Equation of state type: "
diff --git a/src/thermo/SpeciesThermoFactory.cpp b/src/thermo/SpeciesThermoFactory.cpp
index 1a3f125a2..36f957ffe 100644
--- a/src/thermo/SpeciesThermoFactory.cpp
+++ b/src/thermo/SpeciesThermoFactory.cpp
@@ -219,8 +219,8 @@ SpeciesThermo* SpeciesThermoFactory::newSpeciesThermoManager(std::string& stype)
  * Check the continuity of properties at the midpoint
  * temperature.
  */
-void NasaThermo::checkContinuity(std::string name, double tmid, const doublereal* clow,
-                                 doublereal* chigh)
+void NasaThermo::checkContinuity(const std::string& name, double tmid,
+                                 const doublereal* clow, doublereal* chigh)
 {
     // heat capacity
     doublereal cplow = poly4(tmid, clow);
@@ -276,7 +276,7 @@ void NasaThermo::checkContinuity(std::string name, double tmid, const doublereal
  *  @param f0ptr              Ptr to the first XML_Node for the first NASA polynomial
  *  @param f1ptr              Ptr to the first XML_Node for the first NASA polynomial
  */
-static void installNasaThermoFromXML(std::string speciesName,
+static void installNasaThermoFromXML(const std::string& speciesName,
                                      SpeciesThermo& sp, size_t k,
                                      const XML_Node* f0ptr, const XML_Node* f1ptr)
 {
@@ -412,7 +412,7 @@ static doublereal convertDGFormation(size_t k, ThermoPhase* th_ptr)
  *  @param k                  Species index within the phase
  *  @param MinEQ3node         Ptr to the first XML_Node for the first MinEQ3 parameterization
  */
-static void installMinEQ3asShomateThermoFromXML(std::string speciesName,
+static void installMinEQ3asShomateThermoFromXML(const std::string& speciesName,
         ThermoPhase* th_ptr,
         SpeciesThermo& sp, size_t k,
         const XML_Node* MinEQ3node)
@@ -499,7 +499,7 @@ static void installMinEQ3asShomateThermoFromXML(std::string speciesName,
  *  @param f0ptr              Ptr to the first XML_Node for the first NASA polynomial
  *  @param f1ptr              Ptr to the first XML_Node for the first NASA polynomial
  */
-static void installShomateThermoFromXML(std::string speciesName, SpeciesThermo& sp, size_t k,
+static void installShomateThermoFromXML(const std::string& speciesName, SpeciesThermo& sp, size_t k,
                                         const XML_Node* f0ptr, const XML_Node* f1ptr)
 {
     doublereal tmin0, tmax0, tmin1, tmax1, tmin, tmid, tmax;
@@ -565,7 +565,7 @@ static void installShomateThermoFromXML(std::string speciesName, SpeciesThermo&
  *  @param k                  Species index within the phase
  *  @param f                  XML_Node for the SimpleThermo block
  */
-static void installSimpleThermoFromXML(std::string speciesName,
+static void installSimpleThermoFromXML(const std::string& speciesName,
                                        SpeciesThermo& sp, size_t k,
                                        const XML_Node& f)
 {
@@ -595,7 +595,7 @@ static void installSimpleThermoFromXML(std::string speciesName,
  *  @param k                  Species index within the phase
  *  @param tp                 Vector of XML Nodes that make up the parameterization
  */
-static void installNasa9ThermoFromXML(std::string speciesName,
+static void installNasa9ThermoFromXML(const std::string& speciesName,
                                       SpeciesThermo& sp, size_t k,
                                       const std::vector<XML_Node*>& tp)
 {
@@ -649,7 +649,7 @@ static void installNasa9ThermoFromXML(std::string speciesName,
  * Install a stat mech based property solver
  * for species k into a SpeciesThermo instance.
  */
-static void installStatMechThermoFromXML(std::string speciesName,
+static void installStatMechThermoFromXML(const std::string& speciesName,
         SpeciesThermo& sp, int k,
         const std::vector<XML_Node*>& tp)
 {
@@ -702,7 +702,7 @@ static void installStatMechThermoFromXML(std::string speciesName,
  *  @param k                  Species index within the phase
  *  @param f                  XML Node that contains the parameterization
  */
-static void installAdsorbateThermoFromXML(std::string speciesName,
+static void installAdsorbateThermoFromXML(const std::string& speciesName,
         SpeciesThermo& sp, size_t k,
         const XML_Node& f)
 {
diff --git a/src/thermo/StatMech.cpp b/src/thermo/StatMech.cpp
index d990bee14..6bd0f00f5 100644
--- a/src/thermo/StatMech.cpp
+++ b/src/thermo/StatMech.cpp
@@ -35,7 +35,7 @@ StatMech::StatMech()
 StatMech::StatMech(int n, doublereal tlow, doublereal thigh,
                    doublereal pref,
                    const doublereal* coeffs,
-                   std::string my_name) :
+                   const std::string& my_name) :
     m_lowT(tlow),
     m_highT(thigh),
     m_Pref(pref),
diff --git a/src/thermo/StoichSubstanceSSTP.cpp b/src/thermo/StoichSubstanceSSTP.cpp
index 2eddcfabe..00164c532 100644
--- a/src/thermo/StoichSubstanceSSTP.cpp
+++ b/src/thermo/StoichSubstanceSSTP.cpp
@@ -42,7 +42,7 @@ StoichSubstanceSSTP::StoichSubstanceSSTP():
  * @param id     name of the phase id in the file.
  *               If this is blank, the first phase in the file is used.
  */
-StoichSubstanceSSTP::StoichSubstanceSSTP(std::string infile, std::string id) :
+StoichSubstanceSSTP::StoichSubstanceSSTP(const std::string& infile, std::string id) :
     SingleSpeciesTP()
 {
     XML_Node* root = get_XML_File(infile);
@@ -69,7 +69,7 @@ StoichSubstanceSSTP::StoichSubstanceSSTP(std::string infile, std::string id) :
  *  @param phaseRef XML node pointing to a StoichSubstanceSSTP description
  *  @param id       Id of the phase.
  */
-StoichSubstanceSSTP::StoichSubstanceSSTP(XML_Node& xmlphase, std::string id) :
+StoichSubstanceSSTP::StoichSubstanceSSTP(XML_Node& xmlphase, const std::string& id) :
     SingleSpeciesTP()
 {
     if (id != "") {
@@ -446,7 +446,7 @@ void StoichSubstanceSSTP::initThermo()
 }
 
 
-void StoichSubstanceSSTP::initThermoXML(XML_Node& phaseNode, std::string id)
+void StoichSubstanceSSTP::initThermoXML(XML_Node& phaseNode, const std::string& id)
 {
     /*
      * Find the Thermo XML node
@@ -530,7 +530,7 @@ electrodeElectron::electrodeElectron():
  * @param id     name of the phase id in the file.
  *               If this is blank, the first phase in the file is used.
  */
-electrodeElectron::electrodeElectron(std::string infile, std::string id) :
+electrodeElectron::electrodeElectron(const std::string& infile, std::string id) :
     StoichSubstanceSSTP()
 {
     XML_Node* root = get_XML_File(infile);
@@ -557,7 +557,7 @@ electrodeElectron::electrodeElectron(std::string infile, std::string id) :
  *  @param phaseRef XML node pointing to a electrodeElectron description
  *  @param id       Id of the phase.
  */
-electrodeElectron::electrodeElectron(XML_Node& xmlphase, std::string id) :
+electrodeElectron::electrodeElectron(XML_Node& xmlphase, const std::string& id) :
     StoichSubstanceSSTP()
 {
     if (id != "") {
@@ -616,7 +616,7 @@ void electrodeElectron::setParametersFromXML(const XML_Node& eosdata)
     }
 }
 
-void electrodeElectron::initThermoXML(XML_Node& phaseNode, std::string id)
+void electrodeElectron::initThermoXML(XML_Node& phaseNode, const std::string& id)
 {
     doublereal rho = 10.0;
     setDensity(rho);
diff --git a/src/thermo/SurfPhase.cpp b/src/thermo/SurfPhase.cpp
index 9e0e84058..e5bb41c77 100644
--- a/src/thermo/SurfPhase.cpp
+++ b/src/thermo/SurfPhase.cpp
@@ -39,7 +39,7 @@ SurfPhase::SurfPhase(doublereal n0):
     setNDim(2);
 }
 
-SurfPhase::SurfPhase(std::string infile, std::string id) :
+SurfPhase::SurfPhase(const std::string& infile, std::string id) :
     ThermoPhase(),
     m_n0(0.0),
     m_logn0(0.0),
@@ -408,7 +408,7 @@ getCoverages(doublereal* theta) const
 }
 
 void SurfPhase::
-setCoveragesByName(std::string cov)
+setCoveragesByName(const std::string& cov)
 {
     size_t kk = nSpecies();
     compositionMap cc = parseCompString(cov, speciesNames());
diff --git a/src/thermo/ThermoFactory.cpp b/src/thermo/ThermoFactory.cpp
index 36610892e..2484d0a1a 100644
--- a/src/thermo/ThermoFactory.cpp
+++ b/src/thermo/ThermoFactory.cpp
@@ -92,7 +92,7 @@ static int _itypes[]   = {cIdealGas, cIncompressible,
 /*
  * This method returns a new instance of a subclass of ThermoPhase
  */
-ThermoPhase* ThermoFactory::newThermoPhase(std::string model)
+ThermoPhase* ThermoFactory::newThermoPhase(const std::string& model)
 {
 
     int ieos=-1;
@@ -251,7 +251,7 @@ ThermoPhase* newPhase(XML_Node& xmlphase)
     return t;
 }
 
-ThermoPhase* newPhase(std::string infile, std::string id)
+ThermoPhase* newPhase(const std::string& infile, std::string id)
 {
     XML_Node* root = get_XML_File(infile);
     if (id == "-") {
@@ -776,7 +776,7 @@ bool installSpecies(size_t k, const XML_Node& s, thermo_t& th,
  *              containing the species data for that phase.
  *
  */
-const XML_Node* speciesXML_Node(std::string kname,
+const XML_Node* speciesXML_Node(const std::string& kname,
                                 const XML_Node* phaseSpeciesData)
 {
     if (!phaseSpeciesData) {
diff --git a/src/thermo/ThermoPhase.cpp b/src/thermo/ThermoPhase.cpp
index 13dd461ac..f892ad688 100644
--- a/src/thermo/ThermoPhase.cpp
+++ b/src/thermo/ThermoPhase.cpp
@@ -661,7 +661,7 @@ void ThermoPhase::setState_SPorSV(doublereal Starget, doublereal p,
 }
 //=================================================================================================================
 
-doublereal ThermoPhase::err(std::string msg) const
+doublereal ThermoPhase::err(const std::string& msg) const
 {
     throw CanteraError("ThermoPhase","Base class method "
                        +msg+" called. Equation of state type: "+int2str(eosType()));
@@ -775,7 +775,8 @@ SpeciesThermo& ThermoPhase::speciesThermo(int k)
  *            phase. If none is given, the first XML
  *            phase element will be used.
  */
-void ThermoPhase::initThermoFile(std::string inputFile, std::string id)
+void ThermoPhase::initThermoFile(const std::string& inputFile,
+                                 const std::string& id)
 {
 
     if (inputFile.size() == 0) {
@@ -830,7 +831,7 @@ void ThermoPhase::initThermoFile(std::string inputFile, std::string id)
  *             to see if phaseNode is pointing to the phase
  *             with the correct id.
  */
-void ThermoPhase::initThermoXML(XML_Node& phaseNode, std::string id)
+void ThermoPhase::initThermoXML(XML_Node& phaseNode, const std::string& id)
 {
 
     /*
diff --git a/src/thermo/VPSSMgr.cpp b/src/thermo/VPSSMgr.cpp
index 361c85d5a..7b5c028d5 100644
--- a/src/thermo/VPSSMgr.cpp
+++ b/src/thermo/VPSSMgr.cpp
@@ -390,7 +390,7 @@ VPSSMgr::initLengths()
     mPDSS_Vss.resize(m_kk, 0.0);
 }
 
-void VPSSMgr::initThermoXML(XML_Node& phaseNode, std::string id)
+void VPSSMgr::initThermoXML(XML_Node& phaseNode, const std::string& id)
 {
     const PDSS* kPDSS = m_vptp_ptr->providePDSS(0);
     m_p0 = kPDSS->refPressure();
@@ -499,7 +499,7 @@ VPSSMgr_enumType VPSSMgr::reportVPSSMgrType() const
 
 /*****************************************************************/
 
-void VPSSMgr::err(std::string msg) const
+void VPSSMgr::err(const std::string& msg) const
 {
     throw CanteraError("VPSSMgr::" + msg, "unimplemented");
 }
diff --git a/src/thermo/VPSSMgrFactory.cpp b/src/thermo/VPSSMgrFactory.cpp
index d3ff83a6c..0c4dfb01b 100644
--- a/src/thermo/VPSSMgrFactory.cpp
+++ b/src/thermo/VPSSMgrFactory.cpp
@@ -207,7 +207,7 @@ VPSSMgrFactory::~VPSSMgrFactory()
 }
 
 VPSSMgr_enumType
-VPSSMgrFactory::VPSSMgr_StringConversion(std::string ssModel) const
+VPSSMgrFactory::VPSSMgr_StringConversion(const std::string& ssModel) const
 {
     std::string lssModel = lowercase(ssModel);
     VPSSMgr_enumType type;
diff --git a/src/thermo/VPSSMgrFactory.h b/src/thermo/VPSSMgrFactory.h
index 2b4c6680e..1d82e953a 100644
--- a/src/thermo/VPSSMgrFactory.h
+++ b/src/thermo/VPSSMgrFactory.h
@@ -38,8 +38,8 @@ public:
      * @param proc                    Function name error occurred.
      * @param VPSSMgrModel            Unrecognized species thermo calculator name
      */
-    UnknownVPSSMgrModel(std::string proc,
-                        std::string VPSSMgrModel) :
+    UnknownVPSSMgrModel(const std::string& proc,
+                        const std::string& VPSSMgrModel) :
         CanteraError(proc,  "Specified VPSSMgr model "
                      + VPSSMgrModel +
                      " does not match any known type.") {}
@@ -121,7 +121,7 @@ public:
      * @param ssModel  String representing the VPSSMGr object
      */
     virtual VPSSMgr_enumType
-    VPSSMgr_StringConversion(std::string ssModel) const;
+    VPSSMgr_StringConversion(const std::string& ssModel) const;
 
     //! Create a new species variable pressure standard state calculator
     /*!
diff --git a/src/thermo/VPSSMgr_ConstVol.cpp b/src/thermo/VPSSMgr_ConstVol.cpp
index 688d57e9c..9e8733b90 100644
--- a/src/thermo/VPSSMgr_ConstVol.cpp
+++ b/src/thermo/VPSSMgr_ConstVol.cpp
@@ -125,7 +125,7 @@ void VPSSMgr_ConstVol::initThermo()
 }
 
 void
-VPSSMgr_ConstVol::initThermoXML(XML_Node& phaseNode, std::string id)
+VPSSMgr_ConstVol::initThermoXML(XML_Node& phaseNode, const std::string& id)
 {
     VPSSMgr::initThermoXML(phaseNode, id);
 
diff --git a/src/thermo/VPSSMgr_General.cpp b/src/thermo/VPSSMgr_General.cpp
index cb2cabfec..be7eae1ed 100644
--- a/src/thermo/VPSSMgr_General.cpp
+++ b/src/thermo/VPSSMgr_General.cpp
@@ -161,7 +161,7 @@ void VPSSMgr_General::getGibbs_ref(doublereal* g) const
 }
 
 void
-VPSSMgr_General::initThermoXML(XML_Node& phaseNode, std::string id)
+VPSSMgr_General::initThermoXML(XML_Node& phaseNode, const std::string& id)
 {
     VPSSMgr::initThermoXML(phaseNode, id);
 }
diff --git a/src/thermo/VPSSMgr_IdealGas.cpp b/src/thermo/VPSSMgr_IdealGas.cpp
index 71f8eef4e..86f482bee 100644
--- a/src/thermo/VPSSMgr_IdealGas.cpp
+++ b/src/thermo/VPSSMgr_IdealGas.cpp
@@ -90,7 +90,7 @@ void VPSSMgr_IdealGas::_updateStandardStateThermo()
 }
 
 void
-VPSSMgr_IdealGas::initThermoXML(XML_Node& phaseNode, std::string id)
+VPSSMgr_IdealGas::initThermoXML(XML_Node& phaseNode, const std::string& id)
 {
     VPSSMgr::initThermoXML(phaseNode, id);
 }
diff --git a/src/thermo/VPSSMgr_Water_ConstVol.cpp b/src/thermo/VPSSMgr_Water_ConstVol.cpp
index ee09f6f49..99a3a6435 100644
--- a/src/thermo/VPSSMgr_Water_ConstVol.cpp
+++ b/src/thermo/VPSSMgr_Water_ConstVol.cpp
@@ -216,7 +216,7 @@ void VPSSMgr_Water_ConstVol::initThermo()
 }
 
 void
-VPSSMgr_Water_ConstVol::initThermoXML(XML_Node& phaseNode, std::string id)
+VPSSMgr_Water_ConstVol::initThermoXML(XML_Node& phaseNode, const std::string& id)
 {
     VPSSMgr::initThermoXML(phaseNode, id);
 
diff --git a/src/thermo/VPSSMgr_Water_HKFT.cpp b/src/thermo/VPSSMgr_Water_HKFT.cpp
index d3b66649b..46256c091 100644
--- a/src/thermo/VPSSMgr_Water_HKFT.cpp
+++ b/src/thermo/VPSSMgr_Water_HKFT.cpp
@@ -214,7 +214,7 @@ void VPSSMgr_Water_HKFT::initThermo()
 
 
 void
-VPSSMgr_Water_HKFT::initThermoXML(XML_Node& phaseNode, std::string id)
+VPSSMgr_Water_HKFT::initThermoXML(XML_Node& phaseNode, const std::string& id)
 {
     VPSSMgr::initThermoXML(phaseNode, id);
 
diff --git a/src/thermo/VPStandardStateTP.cpp b/src/thermo/VPStandardStateTP.cpp
index 76b115f6b..ff34636d7 100644
--- a/src/thermo/VPStandardStateTP.cpp
+++ b/src/thermo/VPStandardStateTP.cpp
@@ -171,7 +171,7 @@ int VPStandardStateTP::standardStateConvention() const
  */
 
 
-doublereal VPStandardStateTP::err(std::string msg) const
+doublereal VPStandardStateTP::err(const std::string& msg) const
 {
     throw CanteraError("VPStandardStateTP","Base class method "
                        +msg+" called. Equation of state type: "+int2str(eosType()));
@@ -478,7 +478,7 @@ VPStandardStateTP::providePDSS(size_t k) const
  * This routine initializes the lengths in the current object and
  * then calls the parent routine.
  */
-void VPStandardStateTP::initThermoXML(XML_Node& phaseNode, std::string id)
+void VPStandardStateTP::initThermoXML(XML_Node& phaseNode, const std::string& id)
 {
     VPStandardStateTP::initLengths();
 
diff --git a/src/thermo/WaterSSTP.cpp b/src/thermo/WaterSSTP.cpp
index ccfd14ada..28c3d7b07 100644
--- a/src/thermo/WaterSSTP.cpp
+++ b/src/thermo/WaterSSTP.cpp
@@ -40,7 +40,7 @@ WaterSSTP::WaterSSTP() :
 }
 
 
-WaterSSTP::WaterSSTP(std::string inputFile, std::string id) :
+WaterSSTP::WaterSSTP(const std::string& inputFile, const std::string& id) :
     SingleSpeciesTP(),
     m_sub(0),
     m_waterProps(0),
@@ -54,7 +54,7 @@ WaterSSTP::WaterSSTP(std::string inputFile, std::string id) :
 }
 
 
-WaterSSTP::WaterSSTP(XML_Node& phaseRoot, std::string id) :
+WaterSSTP::WaterSSTP(XML_Node& phaseRoot, const std::string& id) :
     SingleSpeciesTP(),
     m_sub(0),
     m_waterProps(0),
@@ -129,7 +129,7 @@ void WaterSSTP::initThermo()
 }
 
 void WaterSSTP::
-initThermoXML(XML_Node& phaseNode, std::string id)
+initThermoXML(XML_Node& phaseNode, const std::string& id)
 {
 
     /*
diff --git a/src/transport/LTPspecies.cpp b/src/transport/LTPspecies.cpp
index fbcb2d282..9e140a13c 100644
--- a/src/transport/LTPspecies.cpp
+++ b/src/transport/LTPspecies.cpp
@@ -21,7 +21,7 @@ public:
     /*!
      *  @param msg    Informative message
      */
-    LTPError(std::string msg) :
+    explicit LTPError(const std::string& msg) :
         CanteraError("LTPspecies", "error parsing transport data: " + msg + "\n") {
     }
 };
@@ -62,7 +62,7 @@ static void getArrhenius(const XML_Node& node,
  *                          is creating a parameterization for (e.g., viscosity)
  *   @param   thermo        const pointer to the ThermoPhase object, which is used to find the temperature.
  */
-LTPspecies::LTPspecies(const XML_Node* const propNode, std::string name,
+LTPspecies::LTPspecies(const XML_Node* const propNode, const std::string& name,
                        TransportPropertyType tp_ind, const thermo_t* thermo) :
     m_speciesName(name),
     m_model(LTP_TD_NOTSET),
@@ -151,7 +151,7 @@ void LTPspecies::adjustCoeffsForComposition()
  *  \verbatim <transport> \endverbatim node and specifies a type of
  *  transport property (like viscosity)
  */
-LTPspecies_Const::LTPspecies_Const(const XML_Node& propNode, std::string name,
+LTPspecies_Const::LTPspecies_Const(const XML_Node& propNode, const std::string& name,
                                    TransportPropertyType tp_ind, const thermo_t* const thermo) :
     LTPspecies(&propNode, name, tp_ind, thermo)
 {
@@ -216,7 +216,7 @@ doublereal LTPspecies_Const::getSpeciesTransProp()
  *   @param   thermo        const pointer to the ThermoPhase object, which is used to find the temperature.
  *
  */
-LTPspecies_Arrhenius::LTPspecies_Arrhenius(const XML_Node& propNode,  std::string name,
+LTPspecies_Arrhenius::LTPspecies_Arrhenius(const XML_Node& propNode, const std::string& name,
         TransportPropertyType tp_ind,  const thermo_t* thermo) :
     LTPspecies(&propNode, name, tp_ind, thermo)
 {
@@ -329,7 +329,7 @@ doublereal LTPspecies_Arrhenius::getSpeciesTransProp()
  *   @param   thermo        const pointer to the ThermoPhase object, which is used to find the temperature.
  *
  */
-LTPspecies_Poly::LTPspecies_Poly(const XML_Node& propNode,  std::string name,
+LTPspecies_Poly::LTPspecies_Poly(const XML_Node& propNode, const std::string& name,
                                  TransportPropertyType tp_ind, const thermo_t* thermo) :
     LTPspecies(&propNode, name, tp_ind, thermo),
     m_temp(-1.0),
@@ -402,7 +402,7 @@ doublereal LTPspecies_Poly::getSpeciesTransProp()
  *   @param   thermo        const pointer to the ThermoPhase object, which is used to find the temperature.
  *
  */
-LTPspecies_ExpT::LTPspecies_ExpT(const XML_Node& propNode, std::string name, TransportPropertyType tp_ind,
+LTPspecies_ExpT::LTPspecies_ExpT(const XML_Node& propNode, const std::string& name, TransportPropertyType tp_ind,
                                  const thermo_t* thermo) :
     LTPspecies(&propNode, name, tp_ind, thermo),
     m_temp(-1.0),
diff --git a/src/transport/LiquidTranInteraction.cpp b/src/transport/LiquidTranInteraction.cpp
index 0387bdd3f..df3373240 100644
--- a/src/transport/LiquidTranInteraction.cpp
+++ b/src/transport/LiquidTranInteraction.cpp
@@ -23,7 +23,7 @@ namespace Cantera
 class LTPError : public CanteraError
 {
 public:
-    LTPError(std::string msg)
+    explicit LTPError(const std::string& msg)
         : CanteraError("LTPspecies",
                        "error parsing transport data: "
                        + msg + "\n") {}
@@ -36,7 +36,7 @@ public:
 class LTPmodelError : public CanteraError
 {
 public:
-    LTPmodelError(std::string msg)
+    explicit LTPmodelError(const std::string& msg)
         : CanteraError("LTPspecies",
                        "error parsing transport data: "
                        + msg + "\n") {}
diff --git a/src/transport/LiquidTransport.cpp b/src/transport/LiquidTransport.cpp
index efa7464b7..2a5f3e145 100644
--- a/src/transport/LiquidTransport.cpp
+++ b/src/transport/LiquidTransport.cpp
@@ -1841,7 +1841,7 @@ void LiquidTransport::stefan_maxwell_solve()
 /*
  * @param msg    String with an informative message
  */
-doublereal LiquidTransport::err(std::string msg) const
+doublereal LiquidTransport::err(const std::string& msg) const
 {
     throw CanteraError("LiquidTransport::err()",
                        "\n\n\n**** Method "+ msg +" not implemented in model "
diff --git a/src/transport/LiquidTransportParams.cpp b/src/transport/LiquidTransportParams.cpp
index abab53815..7f3cb547a 100644
--- a/src/transport/LiquidTransportParams.cpp
+++ b/src/transport/LiquidTransportParams.cpp
@@ -18,7 +18,7 @@ namespace Cantera
 class LTPmodelError : public CanteraError
 {
 public:
-    LTPmodelError(std::string msg) :
+    explicit LTPmodelError(const std::string& msg) :
         CanteraError("LTPspecies", "error parsing transport data: " + msg + "\n") {
     }
 };
diff --git a/src/transport/SimpleTransport.cpp b/src/transport/SimpleTransport.cpp
index 368dca81d..ce1d8d60d 100644
--- a/src/transport/SimpleTransport.cpp
+++ b/src/transport/SimpleTransport.cpp
@@ -1018,7 +1018,7 @@ bool SimpleTransport::update_T()
  * Throw an exception if this method is invoked.
  * This probably indicates something is not yet implemented.
  */
-doublereal SimpleTransport::err(std::string msg) const
+doublereal SimpleTransport::err(const std::string& msg) const
 {
     throw CanteraError("SimpleTransport Class",
                        "\n\n\n**** Method "+ msg +" not implemented in model "
diff --git a/src/transport/TransportBase.cpp b/src/transport/TransportBase.cpp
index 18c6a856e..ab413b456 100644
--- a/src/transport/TransportBase.cpp
+++ b/src/transport/TransportBase.cpp
@@ -121,7 +121,7 @@ void Transport::setThermo(thermo_t& thermo)
 }
 
 
-doublereal Transport::err(std::string msg) const
+doublereal Transport::err(const std::string& msg) const
 {
 
     throw CanteraError("Transport Base Class",
diff --git a/src/transport/TransportFactory.cpp b/src/transport/TransportFactory.cpp
index f33963929..8cbb91a95 100644
--- a/src/transport/TransportFactory.cpp
+++ b/src/transport/TransportFactory.cpp
@@ -68,7 +68,7 @@ public:
      *  @param linenum  inputs the line number
      *  @param msg      String message to be sent to the user
      */
-    TransportDBError(int linenum, std::string msg) :
+    TransportDBError(int linenum, const std::string& msg) :
         CanteraError("getTransportData", "error reading transport data: "  + msg + "\n") {
     }
 };
@@ -355,7 +355,7 @@ LiquidTranInteraction* TransportFactory::newLTI(const XML_Node& trNode,
   make one of several transport models, and return a base class
   pointer to it.
 */
-Transport* TransportFactory::newTransport(std::string transportModel,
+Transport* TransportFactory::newTransport(const std::string& transportModel,
         thermo_t* phase, int log_level)
 {
 
@@ -1476,7 +1476,7 @@ void TransportFactory::fitProperties(GasTransportParams& tr,
  *
  * @ingroup transportProps
  */
-Transport* newTransportMgr(std::string transportModel, thermo_t* thermo, int loglevel, TransportFactory* f)
+Transport* newTransportMgr(const std::string& transportModel, thermo_t* thermo, int loglevel, TransportFactory* f)
 {
     if (f == 0) {
         f = TransportFactory::factory();
diff --git a/src/transport/TransportParams.cpp b/src/transport/TransportParams.cpp
index f4891928a..bf6a448c1 100644
--- a/src/transport/TransportParams.cpp
+++ b/src/transport/TransportParams.cpp
@@ -15,7 +15,7 @@ using namespace std;
 namespace Cantera
 {
 //====================================================================================================================
-NotImplemented::NotImplemented(std::string method) :
+NotImplemented::NotImplemented(const std::string& method) :
     CanteraError("Transport",
                  "\n\n**** Method " + method + " not implemented. ****\n"
                  "(Did you forget to specify a transport model?)\n\n")
diff --git a/src/zeroD/ReactorFactory.cpp b/src/zeroD/ReactorFactory.cpp
index 53eac7977..071fe1241 100644
--- a/src/zeroD/ReactorFactory.cpp
+++ b/src/zeroD/ReactorFactory.cpp
@@ -30,7 +30,7 @@ static int _itypes[]   = {ReservoirType, ReactorType, ConstPressureReactorType,
 /**
  * This method returns a new instance of a subclass of ThermoPhase
  */
-ReactorBase* ReactorFactory::newReactor(string reactorType)
+ReactorBase* ReactorFactory::newReactor(const std::string& reactorType)
 {
 
     int ir=-1;
diff --git a/test_problems/cxx_ex/equil_example1.cpp b/test_problems/cxx_ex/equil_example1.cpp
index c754e6098..7cb59a1ca 100644
--- a/test_problems/cxx_ex/equil_example1.cpp
+++ b/test_problems/cxx_ex/equil_example1.cpp
@@ -28,8 +28,8 @@ void makeEquilDataLabels(const G& gas, V& names)
 }
 
 template<class G, class A>
-void plotEquilSoln(std::string fname, std::string fmt, std::string title,
-                   const G& gas, const A& soln)
+void plotEquilSoln(const std::string& fname, const std::string& fmt,
+                   const std::string& title, const G& gas, const A& soln)
 {
     std::vector<std::string> names;
     makeEquilDataLabels(gas, names);
diff --git a/test_problems/cxx_ex/example_utils.h b/test_problems/cxx_ex/example_utils.h
index 8b9f1295e..7e45cb293 100644
--- a/test_problems/cxx_ex/example_utils.h
+++ b/test_problems/cxx_ex/example_utils.h
@@ -47,8 +47,8 @@ void makeDataLabels(const G& gas, V& names)
 }
 
 template<class G, class A>
-void plotSoln(std::string fname, std::string fmt, std::string title,
-              const G& gas, const A& soln)
+void plotSoln(const std::string& fname, const std::string& fmt,
+              const std::string& title, const G& gas, const A& soln)
 {
     std::vector<std::string> names;
     makeDataLabels(gas, names);
diff --git a/test_problems/cxx_ex/transport_example1.cpp b/test_problems/cxx_ex/transport_example1.cpp
index 0d5fd2b98..52d44e49d 100644
--- a/test_problems/cxx_ex/transport_example1.cpp
+++ b/test_problems/cxx_ex/transport_example1.cpp
@@ -28,8 +28,8 @@ void makeTransportDataLabels(const G& gas, V& names)
 }
 
 template<class G, class A>
-void plotTransportSoln(std::string fname, std::string fmt, std::string title,
-                       const G& gas, const A& soln)
+void plotTransportSoln(const std::string& fname, const std::string& fmt,
+                       const std::string& title, const G& gas, const A& soln)
 {
     std::vector<std::string> names;
     makeTransportDataLabels(gas, names);
diff --git a/test_problems/cxx_ex/transport_example2.cpp b/test_problems/cxx_ex/transport_example2.cpp
index a439dd6b5..e4466b512 100644
--- a/test_problems/cxx_ex/transport_example2.cpp
+++ b/test_problems/cxx_ex/transport_example2.cpp
@@ -29,8 +29,8 @@ void makeTransportDataLabels(const G& gas, V& names)
 }
 
 template<class G, class A>
-void plotTransportSoln(std::string fname, std::string fmt, std::string title,
-                       const G& gas, const A& soln)
+void plotTransportSoln(const std::string& fname, const std::string& fmt,
+                       const std::string& title, const G& gas, const A& soln)
 {
     std::vector<std::string> names;
     makeTransportDataLabels(gas, names);
diff --git a/test_problems/shared/fileLog.h b/test_problems/shared/fileLog.h
index 2b9ccc740..2415b60d7 100644
--- a/test_problems/shared/fileLog.h
+++ b/test_problems/shared/fileLog.h
@@ -6,7 +6,7 @@
 class fileLog: public Cantera::Logger
 {
 public:
-    fileLog(std::string fName) {
+    explicit fileLog(const std::string& fName) {
         m_fName = fName;
         m_fs.open(fName.c_str());
     }