The mechanism of codeposition Of MoO42-, Mo7O246-, and Co2+ ions on the gamma-alumina surface, from aqueous solutions, has been studied in the pH range 4.1-6.1. A mutual promotion in the adsorption of Mo(VI) and Co2+ species was observed in all pH values studied. This was attributed to the strong lateral interactions exerted between the deposited Mo(VI) and Co2+ species. Results obtained from deposition experiments, potentiometric titrations, and microelectrophoresis allowed us to develop and test a codeposition model. The testing involved the following steps : (i) derivation of a number of equations corresponding to the deposition equilibria; (ii) calculation of the amount of the deposited Mo and Co; (iii) calculation of the difference of the hydrogen ions adsorbed on the gamma-alumina surface in the presence and absence of MoO42-, Mo7O246-, and Co2+ ions; (iv) calculation of the zeta-potential of the gamma-alumina particles in a suspension containing the aforementioned Mo(VI) and Co2+ species; (v) comparison of the aforementioned parameters with the corresponding ones determined experimentally. The agreement between the calculated and experimentally determined parameters over the pH range studied allowed us to establish the proposed mechanistic model for the title codeposition. According to this model the deposition of Mo(VI) on the gamma-alumina surface in the presence of Co2+ ions takes place through adsorption of one MoO42- or Mo7O246- ion on a site created by one protonated surface hydroxyl in the inner Helmholtz plane (IHP) of the double layer developed between the support surface and the impregnating solution. The deposition through reaction of one MoO42-ion with two neutral surface hydroxyls, observed previously in the absence of the Co2+ ions in the impregnating suspension, was proved now to be negligible. Moreover, the established codeposition model showed that the extent of adsorption Of Mo7O246-(MoO42- increased (decreased) as pH decreased. Finally, it was found that the amount of the deposited Mo(VI) increased as pH decreased. As regards the Co deposition, it was concluded that it takes place through adsorption of one Co2+ ion on a site created by one deprotonated surface hydroxyl in the IHP. The concentration of the deprotonated surface hydroxyls and thus the amount of the deposited Co increased as pH increased.