Bimodal mesoporous nanocrystalline TiO2 powders were synthesized by a hydrothermal method using tetrabutylorthotitanate as precursor in ethanol-water mixed solutions. The as-prepared TiO2 powders were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and N-2 adsorption-desorption measurements. The photocatalytic activity of the as-prepared TiO2 powders was evaluated by the photocatalytic degradation of acetone under UV-light irradiation at room temperature in air. The effects of alcohol content on the microstructures and photocatalytic activity of the TiO2 powders were investigated and discussed. The molar ratios of EtOH to H2O obviously influenced the crystallization, crystallite size, BET specific surface areas and photocatalytic activity of the prepared TiO2 powders. Hydrothermal treatment enhanced the phase transformation of the TiO2 powders from amorphous to anatase and crystallization. It was found that the photocatalytic activity of TiO2 powders prepared at 150-200 degrees C for 3-24 h in an ethanol-water mixed solution greatly exceeded that of Degussa P25. 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. 2-8 nm and the other inter-aggregated ones with maximum pore diameters of ca. 41-48 nm. (C) 2007 Elsevier B.V. All rights reserved.