Ah&aft--The first step in the search for a general rule has been conducted in order to predict the reaction mechanism in heterogeneous catalysis, based on the mode of the transient response curves of products caused by the concentration jump of the reactants. The response curves based on typical model reactions were. simulated using a digital computer and the simulated responses revealed characteristic modes in accordance with the mechanism of the reactions. The modes can be classified into the following six different types: (I) Instantaneous response: surface reaction controlling or adsorption of reactants controlling. (II) Monotonic response: slow desorption of products or the combination of two or more factors. (III) Overshoot response: slow regeneration of active sites or active species on the surface, or competitive adsorption of reactants. (IV) S-shape response: the slow transformation of certain stable intermediates in the course of the reaction. (V) False start response: self-inhibition of adsorbed reactants to the reaction. And (VI) complex response.: the superposition of two (or more) different types of responses due to dual reaction paths forming the same product or the existence of more than two rate controlling steps. Other factors contributing to the mode of the response curves have also been discussed.