Surface precipitation is a critically important process in many systems of scientific and technological importance, such as modeling the segregation of contaminants from aqueous to solid phases in groundwater and the dispersion of active phases on catalyst supports. XAFS and TEM studies of Co(II) sorbed on Al2O3 indicate clearly that surface precipitates have formed from solutions that are undersaturated with respect to any known bulk solid Co-bearing phase. These studies indicate that the structure of the precipitates is similar to that of Co(OH)2(s), but it is disordered and has a high concentration (about 30%) of Co vacancies. Using the FEFF 6.011 multiple-scattering code, we have been able to model the XANES spectrum of the precipitate phase by making small modifications to the structure of Co(OH)2(s). The formation of this precipitate phase is consistent with the coprecipitation of Co(II) with Al(III) derived from dissolution of the substrate, forming a double hydroxide. The existence of a phase that is less soluble than the Al(OH)3 and Co(OH)2 endmembers was proved by forming a colloidal coprecipitate from a mixture of Al(III) and Co(II) aqueous solutions, at the same pH and concentration as in the sorption samples; this material was shown to have Co-XAFS similar to that of the sorption samples. We conclude, therefore, that the surface precipitation observed in the Co(II)/Al2O3 system is due to the the formation of a ternary hydroxide of Al and Co. Successful quantitative models of metal-ion transport in groundwater should include the possibility of forming ternary and higher order precipitates that include ions derived from sparingly soluble solids. For the impregnation of catalysts, the process of surface coprecipitation will make it more difficult to produce a well dispersed precursor material.