The distributions of Bronsted and Lewis acid sites of different acid strengths on sulfated zirconia calcined at 450-650 degrees C were measured by IR of adsorbed pyridine to elucidate the active sites for butane isomerization. The total numbers of Bronsted acid sites were largest when the catalyst was calcined at 500 degrees C. The total numbers of Lewis acid sites increased with increasing calcination temperature to a maximum at 650 degrees C. The catalytic activity in skeletal isomerization of butane correlated well with the number of Bronsted acid sites but not with the number of Lewis acid sites. The active sites were completely blocked by pyridine irreversibly absorbed at 350 degrees C. We suggest that the strong Bronsted acid sites, which are able to retain pyridine against evacuation at 350 degrees C, act as active sites for butane isomerization on sulfated zirconia.