The acidic properties and 2-methylpentane cracking turnover frequencies (TOFs) were compared for HY and two dealuminated HY zeolites with similar levels of framework Al : HDY that was dealuminated by ammonium hexafluorosilicate, and a (H,NH4)-USY zeolite that was dealuminated by steam. The acid strength distributions were determined by microcalorimetry of NH3 adsorption, and the types of acid sites by FTIR spectroscopy. All zeolites possessed Bronsted acid sites with very few Lewis acid sites. The differential heat of adsorption of NH3 on these zeolites was nearly constant : about 120 kJ/mol for HY and HDY and approximately 7 kJ/mol higher for the (H,NH4)-USY. The cracking TOF for HDY was five times larger than HY, while the TOF of (H,NH4)-USY was 80 times higher These results suggest that the cracking activity may be affected by other factors in addition to acid strength.