diff --git a/src/AbstractState.cpp b/src/AbstractState.cpp
index 13ddd3c6..dec185da 100644
--- a/src/AbstractState.cpp
+++ b/src/AbstractState.cpp
@@ -459,6 +459,10 @@ double AbstractState::keyed_output(parameters key) {
             return PIP();
         case ifundamental_derivative_of_gas_dynamics:
             return fundamental_derivative_of_gas_dynamics();
+        case iTau:
+            return _reducing.T/_T;
+        case iDelta:
+            return _rhomolar/_reducing.rhomolar;
         default:
             throw ValueError(format("This input [%d: \"%s\"] is not valid for keyed_output", key, get_parameter_information(key, "short").c_str()));
     }
diff --git a/src/Tests/CoolProp-Tests.cpp b/src/Tests/CoolProp-Tests.cpp
index 4c0e2eb1..b75f17c2 100644
--- a/src/Tests/CoolProp-Tests.cpp
+++ b/src/Tests/CoolProp-Tests.cpp
@@ -2352,6 +2352,13 @@ TEST_CASE("Github issue #2447", "[2447]") {
     CHECK(err < 0.05);
 }
 
+TEST_CASE("Github issue #2558", "[2558]") {
+    double Tau = CoolProp::PropsSI("Tau", "Dmolar|gas", 200.0, "T", 300.0, "CarbonDioxide[0.5]&Hydrogen[0.5]");
+    double Delta = CoolProp::PropsSI("Delta", "Dmolar|gas", 200.0, "T", 300.0, "CarbonDioxide[0.5]&Hydrogen[0.5]");
+    CHECK(std::isfinite(Tau));
+    CHECK(std::isfinite(Delta));
+}
+
 TEST_CASE("Check methanol EOS matches REFPROP 10", "[2538]"){
     auto TNBP_RP = PropsSI("T", "P", 101325, "Q", 0, "REFPROP::METHANOL");
     auto TNBP_CP = PropsSI("T", "P", 101325, "Q", 0, "HEOS::METHANOL");