On sulfated ZrO2, the comparison of the effects of adsorbing water or ammonia on the infrared bands between 1400 and 1000 cm(-1) suggests that besides structural Lewis sites on the surface of ZrO2, strong Lewis sites are made from chemisorbed SO3. Upon adsorption of water, SO3 is converted, partially, into a surface sulfated species which may act as strong Bronsted sites. At moderate surface hydration, both types of sites may coexist. The catalytic activity in the isomerization of isobutane is a function of the overall nominal surface density in SO4. The acid sites on the surface of phosphated mesoporous zirconia are attributable to surface P-OH groups working as weak Bronsted sites. On both sulfated and phosphated zirconia, surface coating of alumina stabilizes the porosity, but it does not modify the nature of their acid centers.