The method of the reaction order for investigating reaction mechanism, systematically treated by Vetter with reference to electrode processes, is extended applicable not only to charge-transfer-controlled processes but also to those for which a chemical step controls the over-all rate. In order to generalize the method, first, the stoichiometric number of the rate-determining step is explicitly taken into consideration and secondly, all the equations are derived starting with a general rate equation rather than the equation used by Vetter, which is applicable only to charge-transfer-controlled processes. The parameters z +( j) and z −( j) which represent, respectively, numbers of moles of the substance S i needed in constructing the initial and the final systems of the rate-determining step, yield an unambiguous mechanistic conclusion. It is shown from the general analysis that direct evaluation of the z-values is possible irrespective of whether the reaction is controlled by a charge-transfer or a chemical step, but only when the reaction is ideal, ie, the activity coefficient of the critical system of the rate-determining step is independent of the activities of the reacting substances. In non-ideal cases, however, more information on the property of the critical system is needed to evaluate the z-values from experimentally observed reaction-order values. In order to facilitate the evaluation, a criterion is proposed for identifying the reaction-order values with the corresponding z-values. Applications of the method to analyses of several examples are presented.