Periodic density functional theory (DFT) study was performed in order to investigate structural properties as well as electronic states of the sulfated zirconia (S-ZrO2) surface. It was found that H2SO4 dissociatively adsorbs on the tetragonal (101) ZrO2 (t-ZrO2) surface affording H2O and SO3 molecules. The NH3 adsorption analysis was also performed in order to evaluate the acid strength of both Bronsted and Lewis acid sites on the ZrO2 surface. In the case of Bronsted acid sites, the proton on H2O and SO3 co-adsorbed t-ZrO2 surface shows stronger acidity than that of H2O solely adsorbed t-ZrO2 surface. On the H2O and SO3 co-adsorbed tetragonal surface, electron transfer from H2O molecule into ZrO2 surface accompanied by that from ZrO2 surface into SO3 molecule, was observed. The analysis of Lewis acidity indicated that coordinatively unsaturated surface Zr atom which interacted with SO3 molecule showed strong Lewis acidity. (C) 2002 Elsevier Science B.V. All rights reserved.