Upon reducing sulfated zirconia (SZ) catalysts with hydrogen, roughly 50% of the sulfate groups are reduced to sulfur dioxide which is detected mass spectrometrically; the other 50% are reduced to S-= ions that are retained at the surface. The measured ratio H/S of consumed Ii atoms to S atoms originally present as SO4= ions is thus H/S = 5. In the presence of platinum, either deposited on the SZ, or as Pt/NaY in a physical mixture with SZ, or downstream of SZ in a layered bed arrangement, the SO2 is reduced further to H2S, which is detected chemically and by MS. In this case H/S = 8. The TPR peak position is shifted to lower temperature in PtSZ and in physical mixtures of SZ and Pt/NaY. Reduction of sulfate groups lowers the Bronsted acidity of the catalysts, as indicated by the intensity of the LR bands of adsorbed ammonia. Bronsted acidity is almost totally eliminated by hydrogen reduction at 400 degrees C. The position of the band of ammonia on Lewis sites is not significantly affected by sulfate reduction, but the temperature at which ammonia is desorbed from these sites is lowered. The H2S that is formed in the presence of Pt partially poisons the Pt particles; their catalytic signature in the isotope exchange of cyclopentane with D-2 indicates that large Pt ensembles are blocked by adsorbed S atoms even after reduction up to 350 degrees C, but that isolated Pt atoms are still acting as active sites. After complete reduction of the SO4= groups, Pt in PtSZ loses its catalytic activity for the isotope exchange reaction.