The relationship between 2-methylpentane cracking activity and the acid properties of H-Y (acidic Y zeolite), H-USY (acidic ultrastable Y zeolite), steamed H-USY, and (H,NH4)-USY (H,NH4)-ultrastable Y zeolite) was investigated. The acid strength distributions of these samples were determined by microcalorimetry of NH3 adsorption, and the types of acid sites by FTIR spectroscopy. It was found that even for an H-Y sample of a high degree of crystallinity, its cracking activity per unit catalyst weight was 35 times lower than that of H-USY. With further steaming of H-USY, the cracking activity decreased, although the activity per strong Bronsted site remained essentially constant. Interestingly, although the strongly acidic Lewis acid sites were covered by NH3 in (H,NH4)-USY, the catalyst had the same activity as H-USY. Also, the heat of NH3 adsorption on (H,NH4)-USY did not exceed 130 kJ/mol. Thus, it was concluded that strong Lewis acid sites were not active for hydrocarbon cracking, and that 2-methylpentane cracking did not require Bronsted sites with a high heat of NH3 adsorption. H-USY, with both Bronsted and Lewis sites, had a heterogeneous acid strength distribution, whereas zeolites containing only Bronsted sites had a homogeneous acid strength.