TiO2 powder (P25) was hydrothermally treated in pure water at 150 degrees C for different times. The P25 powders before and after hydrothermal treatment were characterized with XRD, TEM, UV-vis, XPS and nitrogen adsorption-desorption isotherms. The results showed that a small amount of anatase was transformed into rutile phase and more aggregates of the TiO2 crystallites formed after hydrothermal treatment, resulting in the decrease of specific surface area of the TiO2 powders. The photocatalytic activity of the hydrothermally treated P25 powders was obviously higher than that of the P25 powder. This can be attributed to the formation of more hydroxyl groups in the surface of TiO2. The more hydroxyl groups would generate more hydroxyl radicals, enhance the adsorption of O-2 molecules and reduce recombination of the photogenerated electrons and holes. Moreover, more hydroxyl was also beneficial to the activation of the rutile phase, leading to an enhanced photocatalytic activity. (c) 2006 Elsevier B.V. All rights reserved.