diff --git a/opm/common/OpmLog/CounterLog.hpp b/opm/common/OpmLog/CounterLog.hpp
index 46c81be4f..241f0dcdb 100644
--- a/opm/common/OpmLog/CounterLog.hpp
+++ b/opm/common/OpmLog/CounterLog.hpp
@@ -33,7 +33,7 @@ namespace Opm {
     class CounterLog : public LogBackend
     {
     public:
-        CounterLog(int64_t messageMask);
+        explicit CounterLog(int64_t messageMask);
         CounterLog();
 
         size_t numMessages(int64_t messageType) const;
diff --git a/opm/common/OpmLog/MessageFormatter.hpp b/opm/common/OpmLog/MessageFormatter.hpp
index 9d69ec75a..c5885a7b5 100644
--- a/opm/common/OpmLog/MessageFormatter.hpp
+++ b/opm/common/OpmLog/MessageFormatter.hpp
@@ -64,7 +64,7 @@ namespace Opm
         }
 
 
-        SimpleMessageFormatter(const bool use_color_coding)
+        explicit SimpleMessageFormatter(const bool use_color_coding)
             : use_color_coding_(use_color_coding)
         {
             prefix_flag_ = Log::MessageType::Warning + Log::MessageType::Error 
diff --git a/opm/common/OpmLog/TimerLog.hpp b/opm/common/OpmLog/TimerLog.hpp
index 5884c07d6..cfabcd1a3 100644
--- a/opm/common/OpmLog/TimerLog.hpp
+++ b/opm/common/OpmLog/TimerLog.hpp
@@ -39,8 +39,8 @@ public:
     static const int64_t StartTimer = 4096;
     static const int64_t StopTimer  = 8192;
 
-    TimerLog(const std::string& logFile);
-    TimerLog(std::ostream& os);
+    explicit TimerLog(const std::string& logFile);
+    explicit TimerLog(std::ostream& os);
 
     void clear();
     ~TimerLog() {};