Adsorption of Co(II) ions on metal oxides is related to radioactive 60Co(II) (de)contamination of nuclear power plants, Co(II) ion retention in soils as a plant nutrient, concentration of Co(II) in deep-sea manganese nodules, and other applications. Here, the amount of adsorbed Co(II) on metal oxides was measured as a function of the pH and concentration of Co(II) ions, and the adsorption properties of metal oxides were evaluated with a model that considers simultaneous (1:1) and (1:2) exchange reactions between Co2+ aqua ions and surface hydroxyl protons obeying the Frumkin isotherm. The possibility of participation of mono- and polynuclear Co(II) hydroxo complexes in the adsorption was examined, and it was suggested that these species play no role under the conditions here. From the model parameters, it was found that the Co2+ adsorption ability of metal oxides increases in the order Al2O3 < Fe2O3 < TiO2 < Fe3O4 < MnO2, showing a good correlation to the electronegativity Xi of the lattice metal ions of the oxides. The Co2+ adsorption was divided into two processes: (1) deprotonation of surface hydroxyl sites and (2) bonding of Co2+ to the deprotonated sites with a negative charge. With increasing Xi, process 1 increases possibly due to the decrease in the donor electron density responsible for covalent bonds with protons, while process 2 changes only slightly. It was suggested that process 2 is due to ionic bond formation ("electrostatic contact adsorption"), which is independent of the donor electron density, and the correlation of the overall process to Xi found here was ascribed to process 1 above. Copyright 1997 Academic Press. Copyright 1997Academic Press