Ultrastable Y (H-USY) zeolite, prepared by steam treatment of Y zeolite, is a very active hydrocarbon cracking catalyst. However, the extent of enhancement in activity compared to a non-steamed sample depends on the reaction condition. A model that has been proposed to explain this behavior is summarized. The model incorporates the three different mechanisms for hydrocarbon cracking, and the dependence of their rates on the partial pressures of reactants and products and temperature. Depending on the reaction condition, such as hydrocarbon pressure, temperature, and conversion, the predominant cracking reaction mechanism may differ. The change in the predominant mechanism may also be a result of the proportionally small increase in external surface area caused by the steaming-induced structural destruction of the zeolite particles. However, these relatively small changes can lead to a much larger overall effect on the cracking rate because of the sensitive dependence of oligomeric cracking, and to a lesser extent, bimolecular cracking on the alkene partial pressure.