The adsorption mechanism of trivalent Ga and In onto gamma-Al2O3 was investigated using a triple-layer model simulation and pressure-jump technique, Bidentate Ga3+ and In3+ and monodentate GaOH2+/InOH2+ are the most likely surface species responsible for Ga(III)/In(III) adsorption, Sorption of Ga(III) and In(III) can be interpreted as an associative process, The adsorption pathway is a two-step mechanism : proton release from surface hydroxyl group(s) followed by coordination of Ga(III)/In(III) species to the depronated site(s), Intrinsic adsorption rate constants cannot be estimated with a liner free-energy relationship between the adsorption rate constant and the rate of water exchange, which is developed solely based on the dissociative sorption mechanism of divalent ions.