The acidity of Cr(III) and Fe(III) (hydr)oxides in water was studied. The experimental point-of zero-charge condition of the particles differs from that of the solution due to solvation effects on ionic adsorption; the surface charge is attributed to polymeric surface complexes. A simple model for the acidity of solid solutions MxRl-xO is presented and contrasted with the complex behavior of ilmenite, which is influenced by the kinetics of FeO leaching and the acid-base properties of the leached layer.