Nitrogen doping of anatase TiO2 powder extended the photocurrent action spectrum for water oxidation from the UV-light region (less than or equal to 400 nm) to the visible-light region (less than or equal to similar to550 nm), as reported. Investigations of the effect of the addition of reductants such as methanol, SCN-, Br-, I-, and hydroquinone to the electrolyte have for the first time given clear experimental evidence to the mechanism that visible-light responses for N-doped TiO2 arise from an N-induced midgap level, formed slightly above the top of the (O-2p) valence band. The investigations, in combination with the above mechanism, have also shown that photocatalytic oxidation of organic compounds on N-doped TiO2, under visible illumination mainly proceed via reactions with surface intermediates of water oxidation or oxygen reduction, not by direct reactions with holes trapped at the N-induced midgap level.