diff --git a/Cantera/cxx/include/Edge.h b/Cantera/cxx/include/Edge.h
index 968990ce9..296a34014 100644
--- a/Cantera/cxx/include/Edge.h
+++ b/Cantera/cxx/include/Edge.h
@@ -33,7 +33,7 @@ namespace Cantera {
         virtual ~Edge() {}
 
         bool operator!() { return !m_ok;}
-        bool ready() { return m_ok; }
+        bool ready() const { return m_ok; }
 
     protected:
         bool m_ok;
diff --git a/Cantera/cxx/include/IncompressibleSolid.h b/Cantera/cxx/include/IncompressibleSolid.h
index eca76f39d..b0160bc78 100644
--- a/Cantera/cxx/include/IncompressibleSolid.h
+++ b/Cantera/cxx/include/IncompressibleSolid.h
@@ -24,7 +24,7 @@ namespace Cantera {
         virtual ~IncompressibleSolid() {}
 
         bool operator!() { return !m_ok;}
-        bool ready() { return m_ok; }
+        bool ready() const { return m_ok; }
 
         //friend std::ostream& operator<<(std::ostream& s, IdealGasMix& mix) {
         //    std::string r = report(mix, true);
diff --git a/Cantera/cxx/include/Interface.h b/Cantera/cxx/include/Interface.h
index 6bc7d46bf..d9b4e575a 100644
--- a/Cantera/cxx/include/Interface.h
+++ b/Cantera/cxx/include/Interface.h
@@ -33,7 +33,7 @@ namespace Cantera {
         virtual ~Interface() {}
 
         bool operator!() { return !m_ok;}
-        bool ready() { return m_ok; }
+        bool ready() const { return m_ok; }
 
     protected:
         bool m_ok;
diff --git a/Cantera/cxx/include/Metal.h b/Cantera/cxx/include/Metal.h
index 87b4533b6..cca906e7e 100644
--- a/Cantera/cxx/include/Metal.h
+++ b/Cantera/cxx/include/Metal.h
@@ -24,7 +24,7 @@ namespace Cantera {
         virtual ~Metal() {}
 
         bool operator!() { return !m_ok;}
-        bool ready() { return m_ok; }
+        bool ready() const { return m_ok; }
 
         //friend std::ostream& operator<<(std::ostream& s, IdealGasMix& mix) {
         //    std::string r = report(mix, true);