Sulfated zirconia has been prepared according to three different procedures, viz., (i) conventional impregnation with sulfuric acid and calcination (3 h at 773 K) of two zirconia's (50 and 217 m(2)/g), (ii) reaction of zirconium tetrachloride with sulfuric acid giving bulk anhydrous zirconium sulfate, and (iii) deposition-precipitation of highly dispersed zirconia on silica and subsequent reaction with either H2S and O-2, or SO2 and O-2, or SO3. The latter two procedures lead to essentially water-free catalysts. Thermogravimetry showed that the impregnated and calcined zirconia's loose sulfate above 830 K (50 m(2)/g) and 910 K (217 m(2)/g). In a gas flow containing water, the sulfated silica-supported zirconia loses sulfate already at 673 K because of the reaction to more volatile sulfuric acid. The catalysts were employed in the gas-phase trans-alkylation of benzene (1) and diethylbenzene (2) to ethylbenzene (3) at 473 and 673 K and in the solvent-free, liquid-phase hydro-acyloxy-addition of acetic acid to camphene (4) to camphene (5) to isobornyl acetate (6) at 338 K. The water-free catalysts were not active; only after addition of water was catalytic activity exhibited. The catalytic activity of the differently prepared sulfated zirconia's is governed by the equilibrium: Zr(SO4)(2) + 4H(2)O Zr(SO4)(2).4H(2)O + nH(2)O ZrO2 + 2H(2)SO(4).aq. Addition of water vapor to the bulk sulfate at 473 K led to the reaction to the tetrahydrate, which was not active, whereas the highly dispersed silica-supported zirconium sulfate reacted to form sulfuric acid. The supported catalyst rapidly released the water at 473 K, which resulted in a rapid drop in catalytic activity. Transport of water through the porous system dominates the activity of the impregnated zirconia's. Accordingly, the slight activity of the zirconia of 50 m(2)/g rapidly dropped at 473 g displayed a high and stable activity. At 673 K, the transport is much K. whereas the zirconia of 217 m(2)/g displayed a high and stable activity. At 673 K, the transport is much more rapid. The activity of the highly porous zirconia was therefore at 673 K much lower than at 473 K. Whereas the gas-phase reaction is governed by transport of water vapor, the liquid-phase reaction is dominated by transport of the reactants to the active sites. Consequently, the sulfated zirconia of 50 m(2)/g showed a considerably higher activity than that of 217 m(2)/g. Also the silica-supported catalyst exhibited a higher activity. The consistent results demonstrate that sulfated zirconia needs water to display activity in the gas-phase and liquid-phase reaction studied.