This article presents a practical approach for estimating adsorption equilibrium constants, K-p, for inclusion in Eley-Rideal type rate expressions, using van’t Hoffs isochore equation, RTlnK(p) = Delta H-ad + T Delta S-ad. The heats of adsorption, -Delta H-ad, have been calculated using proton affinites and heats of condensation, and the entropy of adsorption, Delta S-ad, has been calculated using the Sackur-Tetrode equation. The equilibrium constants have been incorporated into a model for 2-methylpentane cracking that is based on a previously published reaction mechanism. The predicted behavior of the model compares well with published experimental data for a range of catalyst to oil ratios, This new model is also capable of predicting surface coverages of reaction intermediates; for an overall conversion of 10%, over 80% of the catalyst surface is covered with adsorbed C-6 olefin. In addition, the model predictions compare well with published experimental results for dilution of the feed stream with a nonpolar species such as nitrogen.