We conducted a series of phosphate adsorption experiments with alumina and hematite at a range of initial P concentrations and pH values. We found that at low initial P concentrations of < 323 µmole l-1, phosphate adsorption on both alumina and hematite shows a plateau from pH 2.5 to pH 6.5, followed by an abrupt decrease around pH 6.5-7.0. As initial P concentration increases above 500 µmole l-1, the adsorption on alumina displays a sharp maximum around pH 4.0, while the adsorption on hematite exhibits a steep increase with decreased pH but does not seem to reach a maximum even at pH <3.0. Using surface complexation theory, this study demonstrates that the adsorption maximum on alumina results from (1) phosphate being adsorbed preferentially as protonated complexes at high surface P coverage; (2) the electric potential at the oxide surface approaching a plateau with decreased pH as singly coordinated hydroxyls on Al oxides become fully protonated. Fundamentally, however, the adsorption maximum for alumina at pH 4.0 seems to be related to the absence of proton-active but phosphate-inactive triply coordinated hydroxyls at the transformed surface of aluminum oxides.