diff --git a/interfaces/cython/cantera/composite.py b/interfaces/cython/cantera/composite.py
index bcc77c646a..f5fa36afd9 100644
--- a/interfaces/cython/cantera/composite.py
+++ b/interfaces/cython/cantera/composite.py
@@ -317,6 +317,13 @@ def __iadd__(self, other):
             raise ValueError('Cannot add Quantities with different phase '
                 'definitions.')
         assert self.constant == other.constant
+
+        if self.constant == 'HP':
+            dp_rel = 2 * abs(self.P - other.P) / (self.P + other.P)
+            if dp_rel > 1.0e-7:
+                raise ValueError('Cannot add Quantities at constant pressure when'
+                    f'pressure is not equal ({self.P} != {other.P})')
+
         a1,b1 = getattr(self.phase, self.constant)
         a2,b2 = getattr(other.phase, self.constant)
         m = self.mass + other.mass
diff --git a/test/python/test_thermo.py b/test/python/test_thermo.py
index 5d6af0bf18..fd3f9e35c1 100644
--- a/test/python/test_thermo.py
+++ b/test/python/test_thermo.py
@@ -1681,6 +1681,14 @@ def test_add(self):
         self.assertNear(q1.V + q2.V, q3.V)
         self.assertArrayNear(q1.X*q1.moles + q2.X*q2.moles, q3.X*q3.moles)
 
+    def test_add_errors(self):
+        q1 = ct.Quantity(self.gas, mass=5)
+        q2 = ct.Quantity(self.gas, mass=5)
+        q1.constant = q2.constant = 'HP'
+        q2.TP = q1.T, 1.2 * q1.P
+        with pytest.raises(ValueError, match="pressure is not equal"):
+            q3 = q1 + q2
+
     def test_equilibrate(self):
         self.gas.TPX = 300, 101325, 'CH4:1.0, O2:0.2, N2:1.0'
         q1 = ct.Quantity(self.gas)