diff --git a/Cantera/src/ThermoPhase.cpp b/Cantera/src/ThermoPhase.cpp
index d536537d6..ddba8bff1 100644
--- a/Cantera/src/ThermoPhase.cpp
+++ b/Cantera/src/ThermoPhase.cpp
@@ -103,7 +103,7 @@ namespace Cantera {
 	return cAC_CONVENTION_MOLAR;
     }
 
-    void ThermoPhase::getActivities(doublereal* a) {
+    void ThermoPhase::getActivities(doublereal* a) const {
         getActivityConcentrations(a);
         int nsp = nSpecies();
         int k;
diff --git a/Cantera/src/ThermoPhase.h b/Cantera/src/ThermoPhase.h
index 46edd00fd..a34af2c6c 100755
--- a/Cantera/src/ThermoPhase.h
+++ b/Cantera/src/ThermoPhase.h
@@ -542,7 +542,7 @@ namespace Cantera {
      *
      * @param a   Output vector of activities. Length: m_kk.
      */
-    virtual void getActivities(doublereal* a);
+    virtual void getActivities(doublereal* a) const;
     
     //! Get the array of non-dimensional molar-based activity coefficients at
     //! the current solution temperature, pressure, and solution concentration.