Nanocrystalline　TiO2　incorporated　with　praseodymium(III)　nitrate　has　been　prepared　by　an　ultrasound　method　in　a　sol-gel　process.　The　prepared　sample　is　characterized　by　X-ray　diffraction　(XRD),　nitrogen　adsorption　(BET　surface　area),　UV-vis　diffuse　reflectance　spectroscopy　(UV-Vis　DRS)　and　X-ray　photoelectron　spectroscopy　(XPS).　The　prepared　material　consists　of　TiO2　nanocrystalline　with　5　nm　size　incorporated　with　highly　dispersed　Pr(NO3)3.　Visible　light　absorptions　at　444,　469,　482　and　590　nm　are　observed　in　the　prepared　sample.　These　bands　are　attributed　to　the　4f　transitions　3H4　→3P2,　3H4　→3P1,　3H4　→3P0　and　3H4　→1D2　of　praseodymium(III)　ions,　respectively.　This　sample　Pr(NO3)3-TiO2,　as　a　novel　visible　light　photocatalyst,　shows　high　activity　and　stability　in　the　decomposing　rhodamine-B　(RhB)　and　4-chlorophenol　(4-CP)　under　visible　light　irradiation.　Results　examined　by　electron　spin　resonance　spectroscopy　(ESR)　reveal　that　the　irradiation　(>420　nm)　of　the　photocatalyst　dispersed　in　RhB　aqueous　solution　induces　the　generation　of　highly　active　hydroxyl　radicals　({radical　dot}OH),　leading　to　the　cleavage　of　the　whole　conjugated　chromophore　structure　of　RhB.　A　mechanism　based　on　local　excitation　of　praseodymium(III)　nitrate　chromophore　and　interfacial　charge　transfer　from　the　chromophore　to　TiO2　is　proposed　to　explain　the　formation　of　active　hydroxyl　radicals　in　the　photocatalytic　system　under　visible　light　irradiation.　©　2007　Elsevier　B.V.　All　rights　reserved.