This　paper　focuses　on　the　photoactive　centers　of　nitrogen-doped　TiO　2　visible　light　photocatalyst.　A　series　of　N-doped　TiO2　materials　were　prepared　by　a　post-nitridation　route　at　the　temperature　range　of　400-800　°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　spectroscopies.　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+-N　3--Ti4+-V0-]　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　[Ti　4+-N3-]　unit.　©　2010　Elsevier　Inc.　All　rights　reserved.