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We investigated the As(III) and As(V) adsorption complexes forming at the γ-Al₂O₃/water interface as a function of pH and ionic strength (I), using a combination of adsorption envelopes, electrophoretic mobility (EM) measurements, and X-ray absorption spectroscopy (XAS). The As adsorption envelopes show that (1) As(III) adsorption increases with increasing pH and is insensitive to I changes (0.01 and 0.8 M NaNO₃) at pH 3–4.5, while adsorption decreases with increasing I between pH 4.5 and 9.0, and (2) As(V) adsorption decreases with increasing pH and is insensitive to I changes at pH 3.5–10. The EM measurements show that As(III) adsorption does not significantly change the EM values of γ-Al₂O₃ suspension in 0.1 M NaNO₃ at pH 4–8, whereas As(V) adsorption lowered the EM values at pH 4–10. The EXAFS data indicate that both As(III) and As(V) form inner-sphere complexes with a bidentate binuclear …