We report herein a detailed assessment of the roles of O-2, H2O2, (OH)-O-center dot, and O2(-center dot) in the TiO2 assisted photocatalytic oxidation (PCO) of arsenite. Although both arsenite, As(III), and arsenate, As(V), adsorb extensively onto the surface of TiO2, past studies relied primarily on the analysis of the arsenic species in solution, neglecting those adsorbed onto the surface of TiO2, We used extraction and analyses of the arsenic species adsorbed onto the surface of the TiO2 to illustrate that the oxidation of As(III) to As(V) occurs in an adsorbed state during TiO2 PCO. The TiO2 photocatalytic oxidation (PCO) of surface adsorbed As(Ill) in deoxygenated solutions with electron scavengers, Cu2+, and polyoxometalates (POM) yields oxidation rates that are comparable to those observed under oxygen saturation, implying the primary role of oxygen is as a scavenger of the conduction band electron. Pulse radiolysis and competition kinetics were employed to determine a rate constant of 3.6 x 10(6) M-1 s(-1) for the reaction of As(III) with O2(-center dot). Transient absorption studies of adsorbed hydroxyl radicals, generated by subjecting colloidal TiO2 to radiolytic conditions, provide convincing evidence that the adsorbed hydroxyl radical (TiO2+(OH)-O-center dot) plays the central role in the oxidation with As(III) during TiO2 assisted photocatalysis. Our results suggest the reaction of superoxide anion radical does not contribute in the conversion of As(III) when compared to the reaction of As(III) with (OH)-O-center dot radical during TiO2 PCO.