diff --git a/tools/templates/cxx/demo.cpp b/tools/templates/cxx/demo.cpp
index a7f82735f..23ad4ba49 100644
--- a/tools/templates/cxx/demo.cpp
+++ b/tools/templates/cxx/demo.cpp
@@ -36,7 +36,7 @@ void demoprog() {
     IdealGasMix gas("h2o2.cti","ohmech");
     double temp = 1200.0;
     double pres = OneAtm;
-    gas.setState_TPX(temp, pres, "H2:1, O2:1");
+    gas.setState_TPX(temp, pres, "H2:1, O2:1, AR:2");
 
 
     //    Thermodynamic properties