In order to effectively photocatalytically degrade azo dye under solar irradiation, anatase TiO2 that was co-doped with cerium and nitrogen (Ti1-xCexO1-yNy) nanoparticles (NPs) were synthesized using a one-step technique with a modified sol-gel process. The crystal structure and chemical properties were characterized using XRD, BET and XPS. Oxynitride species, Ce4+/Ce3+ pairs, and Ti-O-N and Ti-O-Ce bonds were determined using XPS. The photocatalytic mechanism was investigated through methylene blue (MB) photocatalytic degradation using various filtered wavelengths of light (lambda > 365 nm, lambda > 420 nm, lambda > 500 nm, lambda > 550 nm and lambda > 600 nm) for a period of 10 h. Two experimental parameters were studied systematically, namely the atomic ratio of doped N to Ce and the irradiation wavelength number. The photocatalytic degradation of MB over Ti1-xCexO1-yNy NPs in aqueous suspension was found to follow approximately first-order kinetics according to the Langmuir-Hinshelwood model. The enhanced photocatalytic degradation was attributed to the increased number of photogenerated (OH)-O-center dot radicals. (C) 2009 Elsevier B. V. All rights reserved.