We have developed a kinetic model for isobutane cracking over calcined and steamed Y-zeolite catalysts based on carbo-cation surface chemistry. The model utilized 21 reaction steps, including initiation, oligomerization, beta-scission, olefin desorption, isomerization, and hydride ion transfer, which adequately described the formation of all major products. We estimated kinetic parameters using transition state theory, the Evans-Polanyi correlation, and gas phase thermodynamic data. In order to relate the gas phase calculations to the catalyst surface, we introduced a parameter Delta H-+, which is the heat of stabilization of a carbenium ion relative to the heat of stabilization of a proton in the zeolite framework. The model provided a good description of the experimental data for calcined and steamed catalysts with physically realistic kinetic parameters. The main difference between the two catalysts was the higher Delta H-+ for the steamed catalyst. This indicates that steaming decreased the Bronsted acid strength of the catalyst.