Bimodal nanocrystalline mesoporous TiO2 powders with high photocatalytic activity were prepared by a hydrothermal method using tetrabutylorthotitanate (TiO(C4H9)(4), TBOT) as precursor. The as-prepared TiO, powders were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS) and N-2 adsorption-desorption measurements. The photocatalytic activity of the as-prepared TiO2 powders was evaluated by the photocatalytic degradation of acetone (CH3COCH3) under UV-Iight irradiation at room temperature in air. The effects of hydrothermal temperature and time on the microstructures and photocatalytic activity of the TiO2 powders were investigated and discussed. It was found that hydrothermal treatment enhanced the phase transformation of the TiO2 powders from amorphous to anatase and crystallization of anatase. All TiO2 powders after hydrothermal treatment showed bimodal pore-size distributions in the mesoporous region: one was intra-aggregated pores with maximum pore diameters of ca. 4-8 nm and the other with inter-aggregated pores with maximum pore diameters of ca. 4550 nm. With increasing hydrothermal temperature and time, the average crystallite size and average pore size increased, in contrast, the Brunauer-Emmett-Teller (BET) specific surface areas, pore volumes and porosity steadily decreased. An optimal hydrothermal condition (180 degrees C for 10 h) was determined. The photocatalytic activity of the prepared TiO2 powders under optimal hydrothermal conditions was more than three times higher than that of Degussa P25. (c) 2006 Elsevier B.V. All rights reserved.