TiO2 photocatalysts were prepared by doping transition metal ions like V5+, Mo6+ and an inner transition metal Th4+ in the concentration range of 0.02-0.1% and were characterized by various analytical techniques. The photocatalytic activities of these catalysts were studied for the mineralization of chlorpyrifos (CP) as a probe molecule. X-ray diffraction results showed only anatase phase irrespective of nature, oxidation state and concentration of these dopants. The photocatalytic activity of undoped TiO2 showed Th4+ a better activity under UV light compared to doped catanersoargtnatysts.(0.06%)-TiO2 showed highest activity for the mineralization of CP. This was attributed to the prolonged separation of photogenerated electron-hole pairs, high Specific Surface area of the catalyst and high concentration of surface adsorbed water/hydroxyl groups. Further large shift in the absorption band (460 and 482 nm) due to the creation of mid band gap states by Th4+ dopant and also by its small crystallite size additionally contributes to the enhancement of the degradation process. The degradation pathway was followed by UV-vis spectroscopic and GC-MS analysis. (C) 2009 Elsevier B.V. All rights reserved.