AbstractA theoretical treatment of the decay of activity in aging catalysts is presented. In this treatment it is assumed that catalyst activity is a function of time‐on‐stream only and on the basis of this assumption the behavior of static bed catalytic reactors is studied. From these considerations it is deduced that there are three distinguishable types of behavior in decaying catalysts. These types are described in terms of their characteristic behavior and it is shown how the performance of certain rapidly decaying catalysts can be optimized in plant operat