Temperature programmed reduction, X-ray photoelectron spectroscopy, and X-ray diffraction were used for comparative characterization of Pt on sulfated and nonsulfated zirconia catalysts calcined both in situ and ex situ at different temperatures in the range 25-800-degrees-C. The effect of sulfur species on the state of Pt was twofold. They decreased the interactions between Pt oxide and the support, thus leading to large particles easy to decompose into large metallic Pt particles that resisted reoxidation in air. In the absence of sulfur species, up to a calcination temperature of 650-degrees-C, Pt oxide and metallic Pt particles remained small. Such samples underwent almost complete reoxidation during storage in air. Sulfur species also had a direct effect on the state of Pt. Sulfur dioxide produced by decomposition of sulfate played the role of a reducing agent. Under temperature programmed reduction conditions, two sulfate reduction peaks were obtained regardless of the presence of Pt. The occurrence of two different sulfate species was suggested and a tentative assignment proposed.