This paper focuses on the photoactive centers of nitrogen-doped TiO2 visible light photocatalyst A series of N-doped TiO2 materials were prepared by a post-nitridation route at the temperature range of 400800 C The photocatalytic oxidation of acetone as a model reaction was used to evaluate the photocatalytic properties of the materials The chemical states of doped nitrogen species were characterized by near-edge X-ray absorption fine structure X-ray photoelectron and electron paramagnetic resonance spectroscopes The results reveal that four types of N species exist alone or together in TiO2 depending on nitridation temperature The photoactive centers of the materials are a diamagnetic [O-Ti4+-N3--Ti4+-V-0(-)] cluster containing an oxygen vacancy and a nitrogen anion The visible light photocatalysis of N-doped TiO2 is proposed to be initiated by an excited state of the surface [Ti4+-N3-] unit (C) 2010 Elsevier Inc All rights reserved