ZrO₂/SO₄^-2 and NiO-ZrO₂/SO₄^-2 were prepared by precipitation followed by modification with H₂SO₄. Infrared spectra of sulfate adsorbed on Zr（OH）₄ showed replacement of surface hydroxyls and the formation of adsorbed sulfate groups with C₂ν symmetry. On the basis of the results obtained from X-ray diffraction and DTA for ZrO₂ modified with sulfate ion, the transition temperature from amorphous to tetragonal phase was as high as 200 ℃, as Comared with pure ZrO₂. For NiO-ZrO₂, the phase transition temperature shifted to higher temperature due to the solubility of NiO in the ZrO₂. The sulfate ion adsorbed on the surface was decomposed to release SO₃ at 700-840 ℃ differently depending on the NiO content.