The transient behavior of a heterogeneous catalytic process described by a 2-site adsorption-desorption model was studied. After a stop on the supply of reactant, a temporary increase in the prodn. rate is predicted. Anal. solns. to describe the instantaneous reaction rate in a differential reactor are presented. The theor. results explain published exptl. observations that the reaction rate under transient conditions can be several times higher than under steady-state conditions. By model simulations it was found that the transient rate can be considerably higher than under optimum steady-state conditions. The relations developed are useful in model discrimination and parameter estn. [on SciFinder (R)]