The adsorption and coprecipitation characteristics of Cr(III), Zn(II), and Ni(II) with amorphous hydrous iron(III) oxide (HFO) and those of Zn(II) and Ni(II) with amorphous hydrous chromium(III) oxide (HCO) have been measured by the authors (R. J. Crawford, I. H. Harding, and D. E. Mainwaring, Langmuir 9, 3050, 1993). In the current study, the zeta potentials of these colloidal substrates have been measured as a function of pH during and under the same conditions as the adsorption and coprecipitation processes. The Gouy-Chapman-Stern-Grahame model of the electrical double layer has been used, with allowance for specific site interactions, to accurately model the pH-dependent zeta potential for the individual hydrous metal oxides involved. A surface site mixing model has been proposed which models the zeta potential of the oxide substrates in the presence of an increasing amount of a dissimilar second oxide surface, which results from the process of metal ion adsorption or coprecipitation. This mixing model was found to satisfactorily account for all trends observed in the zeta potentials measured during the adsorption and coprecipitation of Cr(III), Zn(II), and Ni(II) with HFO and those of Zn(II) and Ni(II) with HCO.