diff --git a/opm/core/fluid/RockCompressibility.cpp b/opm/core/fluid/RockCompressibility.cpp
index 7ca7acfb3..be6b73571 100644
--- a/opm/core/fluid/RockCompressibility.cpp
+++ b/opm/core/fluid/RockCompressibility.cpp
@@ -62,7 +62,7 @@ namespace Opm
         return p_.empty() && (rock_comp_ == 0.0);
     }
 
-    double RockCompressibility::poroMult(double pressure)
+    double RockCompressibility::poroMult(double pressure) const
     {
         if (p_.empty()) {
             // Approximating with a quadratic curve.
@@ -73,7 +73,7 @@ namespace Opm
         }
     }
 
-    double RockCompressibility::rockComp(double pressure)
+    double RockCompressibility::rockComp(double pressure) const
     {
         if (p_.empty()) {
             return rock_comp_;
diff --git a/opm/core/fluid/RockCompressibility.hpp b/opm/core/fluid/RockCompressibility.hpp
index 94220d318..a5c9a983b 100644
--- a/opm/core/fluid/RockCompressibility.hpp
+++ b/opm/core/fluid/RockCompressibility.hpp
@@ -45,10 +45,10 @@ namespace Opm
         bool isActive() const;
 
         /// Porosity multiplier.
-        double poroMult(double pressure);
+        double poroMult(double pressure) const;
 
         /// Rock compressibility = (d poro / d p)*(1 / poro).
-        double rockComp(double pressure);
+        double rockComp(double pressure) const;
 
     private:
         std::vector<double> p_;