Isotherms were developed at pH 6.9 for adsorption (ADS) and coprecipitation (CPT) of Cu by hydrous oxides of Fe (HFO) and Al (HAO) to study the role of sorbate/sorbent ratio in metal cation removal. For low sorbate/sorbent conditions, HFO had a higher Cu retention capacity than HAO regardless of contact methodology. For either oxide, CPT was consistently more effective than ADS in removing Cu from solution. At high sorbate/sorbent ratios, surface precipitation dominates and the oxide's net cation retention capacity depends on the nature and solubility of the precipitate formed at the oxide–water interface. X-ray diffraction patterns and isotherms of HAO for both ADS and CPT suggest formation of a solid solution [e.g., CuAl 2 O 4 (s)] with dramatically lower solubility than Cu(OH) 2 (s) precipitated in bulk solution. In contrast, Cu precipitated on the HFO surface exhibited a solubility comparable to the bulk precipitated Cu(OH) 2 (s). Therefore, at high sorbate/sorbent ratios, HAO has a higher Cu “apparent” sorption capacity than HFO. The relative utility of these oxides as metal scavengers thus depends markedly on sorbate/sorbent conditions.