Ammonia stepwise temperature programmed desorption (STPD) and FT-IR spectroscopy have been used to study the acidity of dealuminated β-zeolites with Si/Al ratios in the range of 14.5 to 132. A carefully optimized temperature profile starting at 150°C (in order to exclude physisorbed NH 3 ) revealed five peaks at about 180°C, 250°C, 350°C, 440°C, and 540°C. Our investigations indicate that the first two peaks correspond to Lewis-type acidity of low strength and that the last three peaks correspond to Br nsted-type acidity of increasing strength. A one-to-one relation between the total number of ammonia molecules desorbed in the above temperature range and that of the Al atoms exists. High strength Lewis sites were not observed probably because the extent of dehydroxylation in the above temperature range was minimal. By increasing the level of dealumination, it was found that the relative population of strong Br nsted sites increases in comparison with the weak ones. The present stepwise temperature programmed desorption experiments coupled with FT-IR provide a very accurate tool for the quantitative measurement of the strength of the zeolite acid sites.