Three types of superacidic sulfated zirconia (S-ZrO2) were prepared by different methods using hydrated zirconia and sulfuric acid. Their proton conductivities were evaluated at 20-150 degreesC under saturated water vapor pressure. We found that the SOx concentration on S-ZrO2 varied largely depending on the method of preparation. The S/Zr atomic ratio changed from 0.046 for the sample prepared through a mixture of hydrated zirconia powder and sulfuric acid to 0.35 for the sample prepared through a mixture of hydrated zirconia sol and sulfuric acid. A powder compact of the former S-ZrO2 showed a proton conductivity of 4 x 10(-2) S cm(-1) at 70 degreesC and 8 x 10(-3) S cm(-1) at 150 degreesC, whereas that of the latter S-ZrO2 exhibited a high conductivity of 5 x 10(-2) S cm(-1) at 60-150 degreesC. Our experiment confirmed from IR spectra that there exists SOx bonded tightly with Zr on a heat-treated S-ZrO2 surface. The electronic polarization induced by such SOx was assumed to improve the Lewis and Bronsted acidities on Zr, leading to higher proton conductivities. (C) 2004 Elsevier B.V. All rights reserved.