Phosphate adsorption on aluminum oxides is characterized by a maximum at a pH value around 4.0. Using surface complexation theory, this study demonstrates that the maximum adsorption arises from: 1) protonation of surface hydroxyls; 2) the inner- sphere surface phosphate complexes being fully protonated at pH < 9.0; 3) the surface electric potential changing at an extremely slow (near-zero) rate at pH < 7.0. The maximum of adsorption can be regarded as a typical behavior that arises from protonation of surface hydroxyls if the adsorbed anion is either fully protonated or simply can not be protonated.