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 H-3(4)-> P-3(2), H-3(4)-> P-3(1), H-3(4)-> P-3(0) and H-3(4)-> D-1(2) 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 (degrees 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. (C) 2007 Elsevier B.V. All rights reserved.