Abstract There are many areas in the world where the ground water has been contaminated by arsenic. TiO2 is one of the most promising materials that can remove arsenic from groundwater supplies by the adsorption-based processes. The TiO2 surface is capable of photo-catalytic oxidation (PCO) changing the arsenite [As(III)] to arsenate [As(V)] which is more easily absorbed by the surface, increasing the efficiency of the process. In this paper, a density functional theory calculation has been performed to investigate the adsorption of As(III) on a perfect TiO2 anatase (1 0 1) surface. All the As(III) solution species such as H3AsO3, H2AsO3−, HAsO32− and AsO33− are put onto the surface with many different possible attitudes to obtain the adsorption energy. Based on the adsorption energy and the concentration of H3AsO3, H2AsO3−, HAsO32− and AsO33− in an aqueous solution, the bidentate binuclear (BB) adsorption configurations of H2AsO3− on the surface are more favorable at low As(III) concentrations, whereas BB form and monodentate mononuclear (MM) form may coexist at higher concentrations. By calculating H2AsO3− co-adsorption with water and oxygen, we can confirm the deep acceptor character of an adsorbed O2 molecule which implies that surface superoxide (or hydroperoxyl radical) plays an important role during the PCO process of As(III) on TiO2 surface.