This study aims to establish a comprehensive understanding on the overall mechanism that satisfactorily explains for all the observed results in the photocatalytic oxidation (PCO) of As(III). The key point that has confused the oxidation mechanism is related with the fact that the adsorbed As(III) on TiO2 serves as an external charge-recombination center where the reaction of As(III) with an OH radical (or hole) is immediately followed by an electron transfer to make a null cycle. In addition, As(III) can be efficiently oxidized to As(V) under UV254nm irradiation in the absence of any additives although the molar absorption coefficient of As(III) was measured to be very low. Secondary oxidants generated during the photoreaction are maimly responsible for the homogeneous photolytic oxidation of As(III).