Sulfated zirconia catalysts, with and without Pt, were subjected to heat treatments in air, helium, or a mixture of hydrogen and helium. Oxygen retards the evolution of the sulfur species while hydrogen accelerates their evolution; thus, the temperature for the evolution of sulfur species is higher in air than in helium and in hydrogen. The loss of sulfur allows the exothermic tetragonal phase transition to monoclinic zirconia to occur. From the temperature of the exotherm and the composition of the evolved gases, it appears that the sulfur species that evolve early in the decomposition contain more oxygen than those that evolve during the later part of the decomposition. Only reduced sulfur species are observed during the heating of Pt-SO42--ZrO2 in hydrogen; however, reduced species are observed only during the later portion of the decomposition period for the SO42--ZrO2 sample that does not contain Pt.