diff --git a/include/cantera/transport/UnityLewisTransport.h b/include/cantera/transport/UnityLewisTransport.h
index e4f4c49f8..9ad655212 100644
--- a/include/cantera/transport/UnityLewisTransport.h
+++ b/include/cantera/transport/UnityLewisTransport.h
@@ -64,9 +64,24 @@ public:
         throw NotImplementedError("UnityLewisTransport::getMixDiffCoeffsMole");
     }
 
-    //! Not implemented for unity Lewis number approximation
+    //! Returns the unity Lewis number approximation based diffusion
+    //! coefficients [m^2/s].
+    /*!
+     * These are the coefficients for calculating the diffusive mass fluxes
+     * from the species mass fraction gradients, computed as
+     *
+     * \f[
+     *     D_{km} = \frac{\lambda}{\rho c_p}
+     * \f]
+     *
+     * @param[out] d  Vector of diffusion coefficients for each species (m^2/s).
+     * length m_nsp.
+     */
     virtual void getMixDiffCoeffsMass(double* const d){
-        throw NotImplementedError("UnityLewisTransport::getMixDiffCoeffsMass");
+        double Dm = thermalConductivity() / (m_thermo->density() * m_thermo->cp_mass());
+        for (size_t k = 0; k < m_nsp; k++) {
+            d[k] = Dm;
+        }
     }
 };
 }
diff --git a/interfaces/cython/cantera/test/test_transport.py b/interfaces/cython/cantera/test/test_transport.py
index 0848fad2b..318faa97a 100644
--- a/interfaces/cython/cantera/test/test_transport.py
+++ b/interfaces/cython/cantera/test/test_transport.py
@@ -18,11 +18,16 @@ class TestTransport(utilities.CanteraTest):
     def test_unityLewis(self):
         self.phase.transport_model = 'UnityLewis'
         alpha = self.phase.thermal_conductivity/(self.phase.density*self.phase.cp)
-        Dkm = self.phase.mix_diff_coeffs
+        Dkm_prime = self.phase.mix_diff_coeffs
+
+        Dkm = self.phase.mix_diff_coeffs_mass
 
         eps = np.spacing(1) # Machine precision
         self.assertTrue(all(np.diff(Dkm) < 2*eps))
         self.assertNear(Dkm[0], alpha)
+        self.assertTrue(all(np.diff(Dkm_prime) < 2*eps))
+        self.assertNear(Dkm_prime[0], alpha)
+
 
     def test_mixtureAveraged(self):
         self.assertEqual(self.phase.transport_model, 'Mix')