diff --git a/interfaces/cython/cantera/test/test_purefluid.py b/interfaces/cython/cantera/test/test_purefluid.py
index 610185b3a..32be4e723 100644
--- a/interfaces/cython/cantera/test/test_purefluid.py
+++ b/interfaces/cython/cantera/test/test_purefluid.py
@@ -26,6 +26,22 @@ class TestPureFluid(utilities.CanteraTest):
         self.assertNear(self.water.T, 500)
         self.assertNear(self.water.X, 0.8)
 
+    def test_set_X(self):
+        self.water.TX = 500, 0.0
+        p = self.water.P
+        self.water.X = 0.8
+        self.assertNear(self.water.P, p)
+        self.assertNear(self.water.T, 500)
+        self.assertNear(self.water.X, 0.8)
+
+        self.water.TP = 650, 101325
+        with self.assertRaises(Exception):
+            self.water.X = 0.1
+
+        self.water.TP = 300, 101325
+        with self.assertRaises(Exception):
+            self.water.X = 0.3
+
     def test_set_minmax(self):
         self.water.TP = self.water.min_temp, 101325
         self.assertNear(self.water.T, self.water.min_temp)
diff --git a/interfaces/cython/cantera/thermo.pyx b/interfaces/cython/cantera/thermo.pyx
index e2c391592..be310a554 100644
--- a/interfaces/cython/cantera/thermo.pyx
+++ b/interfaces/cython/cantera/thermo.pyx
@@ -1170,9 +1170,18 @@ cdef class PureFluid(ThermoPhase):
     or a fluid beyond its critical point.
     """
     property X:
-        """Vapor fraction (quality)."""
+        """
+        Get/Set vapor fraction (quality). Can be set only when in the two-phase
+        region.
+        """
         def __get__(self):
             return self.thermo.vaporFraction()
+        def __set__(self, X):
+            if (self.P >= self.critical_pressure or
+                abs(self.P-self.P_sat)/self.P > 1e-4):
+                raise ValueError('Cannot set vapor quality outside the'
+                                 'two-phase region')
+            self.thermo.setState_Psat(self.P, X)
 
     property TX:
         """Get/Set the temperature and vapor fraction of a two-phase state."""