diff --git a/interfaces/cython/cantera/composite.py b/interfaces/cython/cantera/composite.py
index a1d0fd8f1..b7b78e23c 100644
--- a/interfaces/cython/cantera/composite.py
+++ b/interfaces/cython/cantera/composite.py
@@ -235,10 +235,10 @@ class Quantity(object):
         return self
 
     def __mul__(self, other):
-        return Quantity(self.phase, mass=self.mass * other)
+        return Quantity(self.phase, mass=self.mass * other, constant=self.constant)
 
     def __rmul__(self, other):
-        return Quantity(self.phase, mass=self.mass * other)
+        return Quantity(self.phase, mass=self.mass * other, constant=self.constant)
 
     def __iadd__(self, other):
         if (self._id != other._id):
diff --git a/interfaces/cython/cantera/test/test_thermo.py b/interfaces/cython/cantera/test/test_thermo.py
index c383fa609..1dc919b15 100644
--- a/interfaces/cython/cantera/test/test_thermo.py
+++ b/interfaces/cython/cantera/test/test_thermo.py
@@ -1137,6 +1137,16 @@ class TestQuantity(utilities.CanteraTest):
         self.assertNear(q1.entropy * 2.5, q2.entropy)
         self.assertArrayNear(q1.X, q2.X)
 
+    def test_multiply_HP(self):
+        self.gas.TPX = 500, 101325, 'CH4:1.0, O2:0.4'
+        q1 = ct.Quantity(self.gas, mass=2, constant='HP')
+        q2 = ct.Quantity(self.gas, mass=1, constant='HP')
+        q2.equilibrate('HP')
+        q3 = 0.2 * q1 + q2 * 0.4
+        self.assertNear(q1.P, q3.P)
+        self.assertNear(q1.enthalpy_mass, q3.enthalpy_mass)
+        self.assertNear(q2.enthalpy_mass, q3.enthalpy_mass)
+
     def test_iadd(self):
         q0 = ct.Quantity(self.gas, mass=5)
         q1 = ct.Quantity(self.gas, mass=5)