A transient approach has been applied to a system deactivating due to coke formation, in order to determine the kinetics for the main reaction. The method, which was used on a Pt-Sn/Al2O3 catalyst deactivated during propane dehydrogenation, consisted of runs in which the gas composition and temperature were changed in steps. By separating the deactivation from the main reaction kinetics, it was possible to obtain rate equations. A simple power-law model was found to fit the data best. Finally, simulations using the model for the main reaction together with a model for the deactivation mechanism were successfully applied to original data. The paper also discusses problems with non-uniform coke profiles in integral reactors, and errors induced in kinetic parameters by using methods that utilize extrapolation to zero process time when finding mechanistic models.