diff --git a/toolchain/mfc/case_validator.py b/toolchain/mfc/case_validator.py
index de2394cdd..9ad95ec03 100644
--- a/toolchain/mfc/case_validator.py
+++ b/toolchain/mfc/case_validator.py
@@ -645,12 +645,17 @@ def check_phase_change(self):
         relax = self.get("relax", "F") == "T"
         relax_model = self.get("relax_model")
         model_eqns = self.get("model_eqns")
+        num_fluids = self.get("num_fluids")
         palpha_eps = self.get("palpha_eps")
         ptgalpha_eps = self.get("ptgalpha_eps")
 
         if not relax:
             return
 
+        self.prohibit(
+            num_fluids is None or num_fluids < 2,
+            "phase change requires num_fluids >= 2 (liquid = 1, vapor = 2)",
+        )
         self.prohibit(
             (model_eqns not in (2, 3) or (model_eqns == 2 and relax_model not in (5, 6)) or (model_eqns == 3 and relax_model not in (1, 4, 5, 6))),
             "phase change requires model_eqns==2 with relax_model in [5,6] or model_eqns==3 with relax_model in [1,4,5,6]",
diff --git a/toolchain/mfc/test_case_validator.py b/toolchain/mfc/test_case_validator.py
index 922da1580..4cb7fe6c9 100644
--- a/toolchain/mfc/test_case_validator.py
+++ b/toolchain/mfc/test_case_validator.py
@@ -242,6 +242,36 @@ def test_accepts_valid_configuration(self):
         self.assertAccepts(REACTIVE_BURN)
 
 
+class TestPhaseChangeFluidPairing(ConstraintTestCase):
+    MSG = "phase change requires num_fluids >= 2 (liquid = 1, vapor = 2)"
+
+    def test_rejects_single_fluid(self):
+        self.assertRejects({**BASE, "relax": "T", "relax_model": 5}, self.MSG)
+
+    def test_rejects_unset_num_fluids(self):
+        params = {k: v for k, v in TWO_FLUID.items() if k != "num_fluids"}
+        self.assertRejects({**params, "relax": "T", "relax_model": 5}, self.MSG)
+
+    def test_accepts_two_fluids(self):
+        self.assertAccepts({**TWO_FLUID, "relax": "T", "relax_model": 5})
+
+    def test_accepts_three_fluids(self):
+        params = {
+            **TWO_FLUID,
+            "num_fluids": 3,
+            "fluid_pp(3)%gamma": 0.4,
+            "fluid_pp(3)%pi_inf": 0.0,
+            "patch_icpp(1)%alpha_rho(3)": 0.0,
+            "patch_icpp(1)%alpha(3)": 0.0,
+            "relax": "T",
+            "relax_model": 5,
+        }
+        self.assertAccepts(params)
+
+    def test_not_checked_when_disabled(self):
+        self.assertAccepts({**BASE, "relax": "F"})
+
+
 class TestSyntheticTurbulence(ConstraintTestCase):
     """A 2D case with one fully specified forcing zone."""