A vacuum quartz-spring balance was used to determine the nitrogen adsorption isotherms at -196-degrees C of silica-titania, silica-alumina, silica-magnesia and chromia-alumina catalysts. The surface acidities of these acid catalysts were determined by following the desorption of pyridine using an electronic vacuum balance. Good agreement was found between the nitrogen adsorption isotherms determined by the vacuum quartz-spring balance and those followed by conventional volumetric methods. The adsorption of pyridine at 35-degrees C was found to be of the physical type where the sorbed pyridine was completely expelled by prolonged out-gassing at the adsorption temperature. The adsorption of pyridine at 150-degrees C on the catalysts investigated is mainly chemical and the amount of pyridine chemisorbed determines the number of acid sites per unit weight of the catalyst or per unit surface area. The thermal desorption of pyridine was found to be related to the strength of the surface acid sites. A fairly good relationship was found between the surface acid density (number of acid sites per cm2) and the catalytic activity towards isopropanol dehydration.