The　electronic　structure　and　photoactivation　process　in　N-doped　TiO　2　is　investigated.　Diffuse　reflectance　spectroscopy　(DRS),　photoluminescence　(PL),　and　electron　paramagnetic　resonance　(EPR)　are　employed　to　monitor　the　change　of　optical　absorption　ability　and　the　formation　of　N　species　and　defects　in　the　heat-　and　photoinduced　N-doped　TiO　2　catalyst.　Under　thermal　treatment　below　573　K　in　vacuum,　no　nitrogen　dopant　is　removed　from　the　doped　samples　but　oxygen　vacancies　and　Ti　3+　states　are　formed　to　enhance　the　optical　absorption　in　the　visible-light　region,　especially　at　wavelengths　above　500　nm　with　increasing　temperature.　In　the　photoactivation　processes　of　N-doped　TiO　2,　the　DRS　absorption　and　PL　emission　in　the　visible　spectral　region　of　450-700　nm　increase　with　prolonged　irradiation　time.　The　EPR　results　reveal　that　paramagnetic　nitrogen　species　(N　s　.),　oxygen　vacancies　with　one　electron　(V　o　.),　and　Ti　3+　ions　are　produced　with　light　irradiation　and　the　intensity　of　N　s　.　species　is　dependent　on　the　excitation　light　wavelength　and　power.　The　combined　characterization　results　confirm　that　the　energy　level　of　doped　N　species　is　localized　above　the　valence　band　of　TiO　2　corresponding　to　the　main　absorption　band　at　410　nm　of　N-doped　TiO　2,　but　oxygen　vacancies　and　Ti　3+　states　as　defects　contribute　to　the　visible-light　absorption　above　500　nm　in　the　overall　absorption　of　the　doped　samples.　Thus,　a　detailed　picture　of　the　electronic　structure　of　N-doped　TiO　2　is　proposed　and　discussed.　On　the　other　hand,　the　transfer　of　charge　carriers　between　nitrogen　species　and　defects　is　reversible　on　the　catalyst　surface.　The　presence　of　oxygen-vacancy-related　defects　leads　to　quenching　of　paramagnetic　N　s　.　species　but　they　stabilize　the　active　nitrogen　species　N　s　-.　Seeing　the　light:　Nitrogen　species　in　N-doped　TiO　2　act　as　photoactive　centers　for　photogenerated　electron　transfer　to　adjacent　defect　sites.　Their　energy　level　is　localized　above　the　valence　band　of　TiO　2　corresponding　to　the　main　absorption　band　at　410　nm　of　N-doped　TiO　2　(see　picture;　N　s　.=paramagnetic　nitrogen　species,　V　o　.=oxygen　vacancy　with　one　electron).　©　2012　WILEY-VCH　Verlag　GmbH　&　Co.　KGaA,　Weinheim.