The heat of adsorption of normal-paraffins on ZSM-5 is a linear function of the number of carbon atoms on the paraffin n, and is approximately half of the latent heat of condensation The rate of cracking of n-paraffins over ZSM-5 increases exponentially with n, which is consistent with a linear dependence of Delta G degrees with n. As temperature is increased, the cracking rate increases when n is less than 16, but decreases when n is greater than 16. This phenomenon is contrary to normal expectations, and is a consequence of the competition between two effects : the increase of intrinsic kinetics with temperature, and the decrease of adsorption strength and the concentration of active intermediates with temperature. The heat of adsorption becomes larger than the activation energy of the intrinsic kinetics when n = 16.