The role of strong Lewis acid sites in the deactivation of H-USY zeolite during 2-methylpentane cracking was investigated. NH3 adsorption microcalorimetry and FTIR of adsorbed NH3 were used to characterize the acid sites of an H-USY zeolite and another USY sample in which the strong Lewis acid sites were poisoned with ammonia. It was found that poisoning of the Lewis acid sites did not affect the rate of deactivation, the cracking activity, and the distribution of the cracked products. Thus, strong Lewis acid sites are not important in the cracking reaction.