The general form of the rate expression for methanol carbonylation containing the concentrations of catalyst, promoter, reactants and products was derived from the reaction mechanism containing all the species observed during the reaction and the known chemistry based on the assumption that the oxidative addition of CH3I to [Rh(CO)2I2]- is the rate-determining step. The parameters involved in the non-linear rate expressions were estimated from the reaction rate data via non-linear regression method. The activation energy calculated from the value of the reaction rate constant estimated from the model is 14.7 kcal mol-1, which is close to the activation energy of 15.0 kcal mol-1, calculated from the experimental data providing partial evidence in support of the proposed mechanism.