Surface complexation models were used to simulate adsorption (ADS) and coprecipitation (CPT) of copper (Cu) by hydrous oxides of iron (HFO) and aluminum (HAO) over a range of pH and surface-loading conditions. The generalized two-layer model was satisfactory for two very different conditions: (1) low sorbate/sorbent ratios where metal-oxide interaction is adequately described as Cu2+ coordination to surface functional groups and (2) under HFO-CPT conditions which result in extremely high adsorption site density (0.425 mol of sites/mol of Fe). As the sorbate/sorbent ratio is progressively increased, the models must account for metal hydrolysis and surface precipitate formation, and Cu interaction with both hydrous oxides could be fitted over a wide range of surface loadings using a comprehensive surface precipitation model. Similar mass law constants for sorption reactions were used for generalized two-layer and surface precipitation modeling, ADS and CPT conditions, and pH-edge and isotherm data. Corroborating sorption and spectroscopic evidence, modeling indicated that Cu precipitated on HAO, unlike HFO, has a markedly lower K-sp than bulk precipitated Cu(OH)(2)(s). Results also suggest that enhanced Cu removal by CPT was not simply a manifestation of higher surface area.