The acidic property of sulfated zirconia, a so-called solid superacid catalyst, was precisely determined by ammonia temperature-programmed desorption with water vapor treatment and theoretical analysis. The desorption peak from the zirconia support was removed by water vapor treatment, and the generation of two types of acid sites was clarified. One kind of acid site is a Lewis type generated on a submonolayer species of the sulfate covering the surface; the surface concentration of the acid site was 0.5 atoms nm(-2), and the adsorption heat of ammonia was ca. 200 kJ mol(-1). This corresponds to -19 of the H-0 function, demonstrating superacidity. The other type of acid sites generated by loading excess sulfate possessed a high concentration (maximum, 2 nm(-2)), an adsorption heat of ca. 160 kJ mol(-1), an H-0 of -12, and Bronsted nature. The former was active for the Friedel-Crafts-type alkylation of benzene with benzyl chloride in the liquid phase, and the latter was active for the skeletal isomerization of butane in gas phase.