Microcalorimetric measurements and infrared spectroscopy of ammonia adsorption were used to characterize the acidic properties of sulfated zirconia catalysts. Reaction kinetic measurements for n-butane isomerization were conducted over catalysts that were selectively poisoned with controlled amounts of ammonia. Initial heats of ammonia adsorption on the strong acid sites of sulfated zirconia were 150-165 kJ/mol, and these sites contain Bronsted acid and possibly Lewis acid centers. Sulfated zirconia samples that show high activity for the isomerization of n-butane possess Bronsted acid sites of intermediate strength, with differential heats of ammonia adsorption between 125 and 140 kJ/mol. The results of selective poisoning of sulfated zirconia with ammonia confirm that Bronsted acid sites of intermediate strength are active for n-butane isomerization at 423 K while not discounting a possible role of the stronger acid sites.