The transient response of the rate of a surface reaction between gas and solid phases is discussed mathematically. The over‐all reaction is assumed to be the results of two elementary steps: adsorption of reactants and desorption of products. Expressions for the nonsteady rates of the two steps are derived and applied to the case of simultaneous occurrence of both steps. This results in a surface catalyzed reaction. It is shown how the analysis of the transient response permits the determination of some of the fundamental parameters of the elementary steps including the values of the step rate constant, the nature and concentration of the adsorbed intermediates. The contribution of nonsteady state kinetics to the elucidation of basic problems on the reactivity of solid surfaces is outlined.