It is shown theoretically that the experimentally observed shifts of isoelectric points of various oxide surfaces toward more basic pH's in concentrated and mixed-solvent electrolyte solutions can be explained by a model previously used to predict the phenomenon of "coagulation zones" in the stability of colloids with adsorption layers of nonionic polymers. The model assumes the standard solvation energies of ions near a solid-liquid interface to be different from those in the hulk due to changes in the solvent structure. To reproduce the trends of experimental data in aqueous solutions, the solubilities of both cations and anions must be decreased, but for cations that must occur to a smaller extent. In mixed solvents a qualitative agreement with the experimental data is achieved by assuming that there is a water-enriched layer near the oxide surface.