Potassium oxalate-doped TiO2 powders were prepared by the base-catalyzed sol-gel process with TiCl4 as a precursor and hexamethylenetetramine as basic agent. The influence of calcination on the structure and photocatalytic activity of the as-prepared TiO2 samples was investigated. The physical properties of the pre- and post-calcined materials were studied by X-ray diffraction spectroscopy, scanning electron microscopy, specific surface area, Fourier-transformed infrared spectroscopy, photoluminescence spectroscopy, and diffuse reflectance ultraviolet-visible spectroscopy. The photocatalytic activity of the products was evaluated by decolorizing methylene blue solutions and comparing them with a commercial TiO2 powder (Degussa P25). The optimum calcination temperature of 800 degrees C yielded samples with enhanced photocatalytic activity comparable to Degussa P25. On the other hand, the non-calcined TiO2 sample (amorphous phase with small amount of anatase phase) produced a better decolorization efficiency than did commercial P25, due to its excellent adsorptive properties with high concentrations of methylene blue solutions. More significantly, the dye adsorbed onto the surface of the non-calcined TiO2 sample could be removed by treating with hydrogen peroxide solution under UV irradiation. The regenerated TiO2 samples could be reused in the subsequent decolorization of methylene blue solution without a decline in decolorization efficiency compared with freshly prepared TiO2 samples, with even slightly better efficiency. (c) 2008 Elsevier B.V. All rights reserved.