Sulfate is a major component of acid precipitation and a primary contributor of surface freshwater and soil acidification. Understanding the mechanism and capacity of soil retention of is critical for prediction of the effects of increased S deposition. This research focused on the quantitative analysis of SO₄^2- adsorption isotherms by γ-Al₂O₃ and kaolinite. Mathematical analyses for the adsorption isotherms demonstrated that SO₄^2- may not be adsorbed on the d-plane (the diffuse layer plane). Both the outer-sphere (the b-plane) and inner-sphere (the o-plane) complexation models predicted the experimental data for SO₄^2- adsorption very well. The adsorption isotherms were described by the Langmuir, two-site Langmuir, Freundlich, and Temkin equations. Sulfate adsorption was highly pH-dependent, increasing with a decrease in suspension pH. The capacity of SO₄^2- adsorption on γ-Al₂O₃ was five times that observed for kaolinite for all three pH levels examined, indicating a higher reactive site density for γ-Al₂O₃ than for kaolinite.