diff --git a/interfaces/cython/cantera/thermo.pyx b/interfaces/cython/cantera/thermo.pyx
index 8700487fb..3bbe34674 100644
--- a/interfaces/cython/cantera/thermo.pyx
+++ b/interfaces/cython/cantera/thermo.pyx
@@ -571,8 +571,8 @@ cdef class ThermoPhase(_SolutionBase):
         """
         Set the composition to a mixture of *fuel* and *oxidizer* at the
         specified equivalence ratio *phi*, holding temperature and pressure
-        constant. Considers the oxidation of C and H to CO2 and H2O. Other
-        elements are assumed not to participate in oxidation (i.e. N ends up as
+        constant. Considers the oxidation of C to CO2, H to H2O and S to SO2.
+        Other elements are assumed not to participate in oxidation (i.e. N ends up as
         N2)::
 
             >>> gas.set_equivalence_ratio(0.5, 'CH4', 'O2:1.0, N2:3.76')
@@ -614,6 +614,11 @@ cdef class ThermoPhase(_SolutionBase):
             nH = np.array([self.n_atoms(k, 'H') for k in range(self.n_species)])
         else:
             nH = np.zeros(self.n_species)
+          
+        if 'S' in self.element_names:
+            nS = np.array([self.n_atoms(k, 'S') for k in range(self.n_species)])
+        else:
+            nS = np.zeros(self.n_species)
 
         Cf = nC.dot(Xf)
         Co = nC.dot(Xo)
@@ -621,8 +626,10 @@ cdef class ThermoPhase(_SolutionBase):
         Oo = nO.dot(Xo)
         Hf = nH.dot(Xf)
         Ho = nH.dot(Xo)
+        Sf = nS.dot(Xf)
+        So = nS.dot(Xo)
 
-        stoichAirFuelRatio = - (Of - 2*Cf - Hf/2.0) / (Oo - 2*Co - Ho/2.0)
+        stoichAirFuelRatio = - (Of - 2*Cf - 2*Sf - Hf/2.0) / (Oo - 2*Co - 2*So - Ho/2.0)
         Xr = phi * Xf + stoichAirFuelRatio * Xo
         self.TPX = None, None, Xr