Titanium dioxide/carbon nanotubes (TiO2/CNTs) composites were prepared with the aid of ultrasonic irradiation. Products of different TiO2:CNTs molar ratio were characterized by X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), Brunauer-Emmett-Teller (BET) adsorption analysis, thermogravimetric and differential thermal analysis (TGA-DSC), photoluminescence (PL) and UV-vis spectroscopy measurements (UV-vis). The photocatalytic activity was evaluated by the degradation of acetone and by the detection of the hydroxyl radical (-OH) signals using electron paramagnetic resonance (EPR), It is found that the crystalline TiO2 is Composed of both anatase and brookite phases. The agglomerated morphology and the particle size of TiO2 in the composites change in the presence of CNTs. The CNTs in the composites are virtually all covered by TiO2. Other than an increase of the surface area, the addition of CNTs does not affect the mesoporous nature of the TiO2. Meanwhile, more hydroxyl groups are available on the surface of the composite than in the case of the pure TiO2. The higher the content of CNTs, there is more effective in the suppression of the recombination of photo-generated e(-)/h(+) pairs. However, excessive CNTs also shield the TiO2 from absorbing UV light. The optimal amount of TiO2 and CNTs is in the range of 1:0.1 and 1:0.2 (feedstock molar ratio). These samples have much more highly photocatalytic activity than P25 and TiO2/activated carbon (AC) composite. The mechanism for the enhanced photocatalytic activity of TiO2 by CNTs is proposed. (c) 2005 Elsevier B.V. All rights reserved.