Mesoporous titanium dioxide nanocrystalline powders were synthesized by ultrasonic-induced hydrolysis reaction of tetrabutyl titanate (Ti(OC4H9)(4)) in pure water without using any templates or surfactants. The as-prepared samples were characterized with X-ray diffraction (XRD) and N-2 adsorption-desorption measurements. The photocatalytic activity of the samples was evaluated using the probe reaction: the photocatalytic oxidation of mixture of formaldehyde and acetone at room temperature. It was found that the as-prepared products by the ultrasonic method were composed of anatase and brooktie phases. The photocatalytic activity of the samples prepared by ultrasonic method is higher than that of commercial Degussa P25 and the samples prepared by conventional hydrolysis method. (C) 2004 Elsevier B.V. All rights reserved.