Magnesium and barium doped TiO2 nanoparticles were synthesized by sol-gel method. The materials were characterized by XRD, BET, FT-IR, TGA, UV-vis, SEM and TEM techniques. The pure TiO2 nanoparticles contained both anatase and rutile phases together, but Mg2+ or Ba2+ metal ion doped nano TiO2 gave only anatase phase. The framework substitution of Mg2+ in nano TiO2 was established by XRD and FT-IR techniques. However, Ba (2+) was retained only on the surface of nano TiO2 as BaCO3 and the absence of framework substitution of Ba2+ in nano TiO2 was evident from XRD and FT-IR analysis. The band gap values of Mg2+ and Ba2+ doped nano TiO2 were higher than the pure nano TiO2 The presence of anatase type structure in nano TiO2 with high crystallinity and high phase stability even after annealing at 800 degrees C substantially indicates that the dopants might inhibit densification and crystallite growth in nano TiO2, by providing dissimilar boundaries. The photocatalytic activity in the degradation of 4-chlorophenol was found to be higher for Mg2+ and Ba (2+) doped nano TiO2 than both pure nano TiO2 and commercial TiO2 (Degussa P-25). The influence of various parameters such as initial concentration of 4-chlorophenol, catalyst loading, pH and light intensity were optimized to obtain maximum degradation. (C) 2007 Elsevier B.V. All rights reserved.