The population of coordinatively unsaturated (cus) Zr4+ ions acting as strong Lewis acid sites on superacid sulfated zirconia catalysts, as well as the relevant heats of adsorption, were measured by means of CO adsorption at 303 K. The presence of charge-withdrawing SO4 groups slightly enhances the electron-accepting ability of the residual free surface cations, not involved in sulfate/metal complexes. H2O adsorbed at 303 K by a stepwise procedure progressively eliminates the Lewis acidic sites, through a mechanism that suggests a competition in strongly binding H2O between cus Zr4+ cations and SO4 groups. Quantitative and energetic data are reported and correlated to IR spectroscopic and catalytic data previously reported on the same catalysts. A model for the catalytic active site is proposed, based on the assumption that a strict cooperation between Lewis and Bronsted acidic sites is likely to exist.