X-ray diffraction, differential thermal analysis and specific surface area measurements have been employed to understand the structural properties of ZrO2 and sulphate-modified ZrO2 calcined at different temperatures. Calcination facilitates the transformation of ZrO2 from cubic to monoclinic phase. Addition of SO4 ion to Zr(OH)(4) helps to retain the cubic phase up to 550 degrees C, and to maintain the surface area through a surface complex formation mechanism. The modified ZrO2 acts as a superacid catalyst and enhances the vapour-phase aniline alkylation.