diff --git a/doc/sphinx/yaml/phases.rst b/doc/sphinx/yaml/phases.rst
index db4d7adf9..fe9f40af8 100644
--- a/doc/sphinx/yaml/phases.rst
+++ b/doc/sphinx/yaml/phases.rst
@@ -538,6 +538,9 @@ The ideal gas model as
 The ideal gas model, using variable pressure standard state methods as
 `described here <https://cantera.org/documentation/dev/doxygen/html/dc/ddb/classCantera_1_1IdealSolnGasVPSS.html#details>`__.
 
+.. deprecated:: 2.5
+   Use the ``ideal-gas`` model instead.
+
 
 .. _sec-yaml-ideal-molal-solution:
 
diff --git a/include/cantera/thermo/IdealSolnGasVPSS.h b/include/cantera/thermo/IdealSolnGasVPSS.h
index 4e4639a1a..197d6f9c4 100644
--- a/include/cantera/thermo/IdealSolnGasVPSS.h
+++ b/include/cantera/thermo/IdealSolnGasVPSS.h
@@ -23,6 +23,9 @@ namespace Cantera
  *
  * An ideal solution or an ideal gas approximation of a phase. Uses variable
  * pressure standard state methods for calculating thermodynamic properties.
+ *
+ * @deprecated "Gas" mode to be removed after Cantera 2.5. Use IdealGasPhase for
+ *     ideal gas phases instead.
  */
 class IdealSolnGasVPSS : public VPStandardStateTP
 {
@@ -62,7 +65,11 @@ public:
     }
 
     //! Set this phase to represent an ideal gas
-    void setGasMode() { m_idealGas = true; }
+    void setGasMode() {
+        warn_deprecated("IdealSolnGasVPSS::setGasMode",
+            "To be removed after Cantera 2.5. Use class IdealGasPhase instead.");
+        m_idealGas = true;
+    }
 
     //! Set this phase to represent an ideal liquid or solid solution
     void setSolnMode() { m_idealGas = false; }
diff --git a/interfaces/cython/cantera/test/test_convert.py b/interfaces/cython/cantera/test/test_convert.py
index 4dac506c8..51d5e0c70 100644
--- a/interfaces/cython/cantera/test/test_convert.py
+++ b/interfaces/cython/cantera/test/test_convert.py
@@ -1187,7 +1187,9 @@ class ctml2yamlTest(utilities.CanteraTest):
             Path(self.test_data_dir).joinpath("pdss_hkft.xml"),
             Path(self.test_work_dir).joinpath("pdss_hkft.yaml"),
         )
-
+        # @todo Remove "gas" mode test after Cantera 2.5 - "gas" mode of class
+        #     IdealSolnGasVPSS is deprecated
+        ct.suppress_deprecation_warnings()
         for name in ["vpss_gas_pdss_hkft_phase", "vpss_soln_pdss_hkft_phase"]:
             ctmlPhase = ct.ThermoPhase("pdss_hkft.xml", name=name)
             yamlPhase = ct.ThermoPhase("pdss_hkft.yaml", name=name)
@@ -1200,6 +1202,7 @@ class ctml2yamlTest(utilities.CanteraTest):
             self.assertEqual(ctmlPhase.element_names, yamlPhase.element_names)
             self.assertEqual(ctmlPhase.species_names, yamlPhase.species_names)
             self.checkThermo(ctmlPhase, yamlPhase, [300, 500])
+        ct.make_deprecation_warnings_fatal()
 
     def test_lattice_solid(self):
         ctml2yaml.convert(
diff --git a/test/data/thermo-models.yaml b/test/data/thermo-models.yaml
index beb17c401..b7007583a 100644
--- a/test/data/thermo-models.yaml
+++ b/test/data/thermo-models.yaml
@@ -93,6 +93,8 @@ phases:
   species: [KCl(l)]
   thermo: Margules
 
+# todo: Remove after Cantera 2.5 - the "gas" moode of class IdealSolnGasVPSS is
+# deprecated.
 - name: IdealSolnGas-gas
   thermo: ideal-gas-VPSS
   species: [{gas-species: [H2O, H2, O2]}]
diff --git a/test/thermo/phaseConstructors.cpp b/test/thermo/phaseConstructors.cpp
index 81179f06f..3d56d9200 100644
--- a/test/thermo/phaseConstructors.cpp
+++ b/test/thermo/phaseConstructors.cpp
@@ -344,8 +344,11 @@ TEST_F(ConstructFromScratch, RedlichKwongMFTP_missing_coeffs)
     EXPECT_THROW(p.setState_TP(300, 200e5), CanteraError);
 }
 
+//! @todo Remove after Cantera 2.5 - "gas" mode of IdealSolnGasVPSS is
+//!     deprecated
 TEST_F(ConstructFromScratch, IdealSolnGasVPSS_gas)
 {
+    suppress_deprecation_warnings();
     IdealSolnGasVPSS p;
     p.addSpecies(sH2O);
     p.addSpecies(sH2);
@@ -367,6 +370,7 @@ TEST_F(ConstructFromScratch, IdealSolnGasVPSS_gas)
     EXPECT_NEAR(p.temperature(), 479.929, 1e-3); // based on h2o2.cti
     EXPECT_NEAR(p.moleFraction("H2O"), 0.01, 1e-4);
     EXPECT_NEAR(p.moleFraction("H2"), 0.0, 1e-4);
+    make_deprecation_warnings_fatal();
 }
 
 TEST(PureFluidFromScratch, CarbonDioxide)
diff --git a/test/thermo/thermoFromYaml.cpp b/test/thermo/thermoFromYaml.cpp
index 856d96dcf..eb35b7403 100644
--- a/test/thermo/thermoFromYaml.cpp
+++ b/test/thermo/thermoFromYaml.cpp
@@ -234,9 +234,13 @@ TEST(ThermoFromYaml, IonsFromNeutral_fromString)
     EXPECT_NEAR(mu[1], -2.88157316e+06, 1e-1);
 }
 
+//! @todo Remove after Cantera 2.5 - "gas" mode of IdealSolnGasVPSS is
+//!     deprecated
 TEST(ThermoFromYaml, IdealSolnGas_gas)
 {
+    suppress_deprecation_warnings();
     auto thermo = newThermo("thermo-models.yaml", "IdealSolnGas-gas");
+    make_deprecation_warnings_fatal();
     thermo->equilibrate("HP");
     EXPECT_NEAR(thermo->temperature(), 479.929, 1e-3); // based on h2o2.cti
     EXPECT_NEAR(thermo->moleFraction("H2O"), 0.01, 1e-4);
diff --git a/test_problems/VPsilane_test/silane_equil.cpp b/test_problems/VPsilane_test/silane_equil.cpp
index f1cf5aa47..98f0be48a 100644
--- a/test_problems/VPsilane_test/silane_equil.cpp
+++ b/test_problems/VPsilane_test/silane_equil.cpp
@@ -9,12 +9,15 @@
 using namespace std;
 using namespace Cantera;
 
+//! @todo Remove this test after Cantera 2.5 - "gas" mode of class
+//!     IdealSolnGasVPSS is deprecated
 int main(int argc, char** argv)
 {
 #if defined(_MSC_VER) && _MSC_VER < 1900
     _set_output_format(_TWO_DIGIT_EXPONENT);
 #endif
     try {
+        suppress_deprecation_warnings();
         IdealSolnGasVPSS gg("silane.xml", "silane");
         ThermoPhase* g = &gg;
         cout.precision(4);