diff --git a/ext/tpx/CarbonDioxide.cpp b/ext/tpx/CarbonDioxide.cpp
index 51a089085..54c85c8fd 100755
--- a/ext/tpx/CarbonDioxide.cpp
+++ b/ext/tpx/CarbonDioxide.cpp
@@ -239,7 +239,7 @@ double CarbonDioxide::up() {
       sum += G[i]*(pow(T,i) - pow(To,i))/double(i);
 
 
-   for (i=0; i<=6; i++) {
+   for (int i=0; i<=6; i++) {
        sum += I(i,egrho, Gamma) *
            ( C(i, Tinverse, T2inverse, T3inverse, T4inverse) - T*Cprime(i,T2inverse, T3inverse, T4inverse) );   
    }
diff --git a/ext/tpx/Heptane.cpp b/ext/tpx/Heptane.cpp
index 8824f2d71..59b63c416 100755
--- a/ext/tpx/Heptane.cpp
+++ b/ext/tpx/Heptane.cpp
@@ -198,7 +198,7 @@ double Heptane::up() {
 
    sum += G[0]*log(T/To);   
 
-   for (i=0; i<=6; i++) {
+   for (int i=0; i<=6; i++) {
       sum += (C(i, Tinverse, T2inverse, T3inverse, T4inverse) - T*Cprime(i,T2inverse, T3inverse, T4inverse))*I(i,egrho, Gamma);   
    }