Ab initio calculations using small sets of basis functions and pseudopotentials within the framework of the cluster model have been applied to the determination of a scale of acidity at oxide surfaces. Results are compared with available experimental data and correlated to values deduced from semiempirical models. A fair agreement between the series of data supports the validity of local models for evaluating acidic properties on oxide surfaces. The acidity of the surface sites of oxides is driven by the electrostatics of the system and is mainly governed by three structural parameters : the coordination number of the hydroxyl group, the charge on the cation(s), and the coordination number of the cation(s).