The reaction : H2O => H2 + 0.5 O2 will by default have forward reaction rate: r = k[H2O] and backward rate r = k[H2]^-0.7 [O2]^1.05, where k = A exp(E/RT) T^n The trick here is to set the forward preexponential factor equal to A *[H2]^-0.7 [O2]^1.05 This can be done with an expression containing the product molar concentrations as below: A= 1.38099E+19 [m^1.050 s^-1 mol^-0.350]*1[K^-n]*(H2.Molar Concentration)^-0.7*(O2.Molar Concentration)^1.05 The dependency of the rate on H2O will occur naturally as a result of CFX’s interpretation of forward rate.