diff --git a/spatialpy/core/reaction.py b/spatialpy/core/reaction.py
index cad800c9..e8591195 100644
--- a/spatialpy/core/reaction.py
+++ b/spatialpy/core/reaction.py
@@ -293,7 +293,7 @@ def _create_mass_action(self):
                 reactant = reactant.name
             # Case 1: 2X -> Y
             if stoichiometry == 2:
-                propensity_function = f"0.5 * {propensity_function} * {reactant} * ({reactant} - 1) / vol"
+                propensity_function = f"{propensity_function} * {reactant} * ({reactant} - 1) / vol"
                 ode_propensity_function += f" * {reactant} * {reactant}"
             else:
                 # Case 3: X1, X2 -> Y;
diff --git a/test/unit_tests/test_reaction.py b/test/unit_tests/test_reaction.py
index d32e51b5..3220fb22 100644
--- a/test/unit_tests/test_reaction.py
+++ b/test/unit_tests/test_reaction.py
@@ -450,7 +450,7 @@ def test__create_mass_action__2X_to_Y(self):
         self.valid_ma_reaction.reactants = {"X": 2}
         self.valid_ma_reaction.products = {"Y": 1}
         self.valid_ma_reaction._create_mass_action()
-        self.assertEqual(self.valid_ma_reaction.propensity_function, "((((0.5*k1)*X)*(X-1))/vol)")
+        self.assertEqual(self.valid_ma_reaction.propensity_function, "(((k1*X)*(X-1))/vol)")
         self.assertEqual(self.valid_ma_reaction.ode_propensity_function, "((k1*X)*X)")
 
     def test__create_mass_action__species_obj_reactant(self):
@@ -581,7 +581,7 @@ def test_add_reactant__massaction_reaction_stoich_0_to_2(self):
         self.valid_ma_reaction.add_reactant("X", 2)
         self.assertIn("X", self.valid_ma_reaction.reactants)
         self.assertEqual(self.valid_ma_reaction.reactants["X"], 2)
-        self.assertEqual(self.valid_ma_reaction.propensity_function, "((((0.5*k1)*X)*(X-1))/vol)")
+        self.assertEqual(self.valid_ma_reaction.propensity_function, "(((k1*X)*(X-1))/vol)")
         self.assertEqual(self.valid_ma_reaction.ode_propensity_function, "((k1*X)*X)")
 
     def test_add_reactant__invalid_species(self):