Concentrations of Bronsted and Lewis acid sites on sulfated-zirconia catalysts were determined using the P-31 MAS NMR spectra of adsorbed trimethylphospine. A sample that had been calcined and exposed to air for a long period exhibited only Bronsted acidity; however, treatment of the sample at progressively higher temperatures resulted in the development of at least three types of Lewis acidity, along with a decrease in the concentration of Bronsted acid sites. In a related study the activity of these catalysts for the alkylation of isobutane with 2-butene was determined. The aged catalyst was inactive, but activation of the material at 100-degrees-C resulted in the most active catalyst. Thermal treatment at higher temperatures resulted in a loss in activity which paralleled the decrease in the Bronsted acid sites. These results are consistent with a model in which strong Bronsted acidity is a result of the interaction between bisulfate groups and adjacent Lewis acid sites.