diff --git a/Cantera/cxx/include/IdealGasMix.h b/Cantera/cxx/include/IdealGasMix.h
index d8681f617..ebb3f1ade 100644
--- a/Cantera/cxx/include/IdealGasMix.h
+++ b/Cantera/cxx/include/IdealGasMix.h
@@ -33,7 +33,7 @@ namespace Cantera {
         virtual ~IdealGasMix() {}
 
         bool operator!() { return !m_ok;}
-        bool ready() { return m_ok; }
+        bool ready() const { return m_ok; }
         friend ostream& operator<<(ostream& s, IdealGasMix& mix) {
             string r = report(mix, true);
             s << r;
diff --git a/Cantera/cxx/include/PureFluid.h b/Cantera/cxx/include/PureFluid.h
index f95b0328f..454fb7410 100644
--- a/Cantera/cxx/include/PureFluid.h
+++ b/Cantera/cxx/include/PureFluid.h
@@ -34,7 +34,7 @@ namespace Cantera {
         virtual ~PureFluid() {}
 
         bool operator!() { return !m_ok;}
-        bool ready() { return m_ok; }
+        bool ready() const { return m_ok; }
         friend ostream& operator<<(ostream& s, PureFluid& mix) {
             string r = report(mix, true);
             s << r;