diff --git a/dumux/material/fluidsystems/brineco2.hh b/dumux/material/fluidsystems/brineco2.hh
index 42662e751257e17d93b490ec7e2d6c56473a0341..423d8cab7790e2a64f66440f01e5af411b258f7b 100644
--- a/dumux/material/fluidsystems/brineco2.hh
+++ b/dumux/material/fluidsystems/brineco2.hh
@@ -218,6 +218,21 @@ public:
         return phaseIdx == gasPhaseIdx;
     }
 
+    /*!
+     * \brief Returns true if and only if a fluid phase is assumed to
+     *        be an ideal gas.
+     * \param phaseIdx The index of the fluid phase to consider
+     */
+    static constexpr bool isIdealGas(int phaseIdx)
+    {
+        assert(0 <= phaseIdx && phaseIdx < numPhases);
+        // let the fluids decide
+        if (phaseIdx == gasPhaseIdx)
+            return useConstantSalinity ? (ConstantSalinityBrine::gasIsIdeal() && CO2::gasIsIdeal())
+                                       : (H2O::gasIsIdeal() && CO2::gasIsIdeal());
+        return false; // not a gas
+    }
+
     /*!
      * \brief Returns true if and only if a fluid phase is assumed to
      *        be an ideal mixture.