Quantum chemical calculations were performed to estimate relative Gibbs free energies of sulfate adsorption on variably charged Al- and Fe-(hydr)oxides.  Inner-sphere bidentate bridging and monodentate adsorption was predicted to be exothermic on positively charged Al- and Fe-(hydr)oxides (range -19 to -124 kJ mol^-1) .  However, inner-sphere and H-bonded adsorption on neutral Al- and Fe-(hydr)oxides was predicted to be endothermic (range +5 to +61 kJ mol^-1).  At highest positive surface charge, bidentate bridging adsorption was most thermodynamically favorable.  At intermediate positive surface charge, bidentate bridging and monodentate adsorption were thermodynamically equivalent on Al-(hydr)oxides; monodentate adsorption was more thermodynamically favorable on Fe-(hydr)oxides compared with bidentate bridging adsorption.  Predicted thermodynamic behavior of sulfate adsorption on Al- and Fe-(hydr)oxides qualitatively agreed with experimental pH-dependent surface charge adsorption measurements (i.e. adsorption edge).