A series of cerium ion-doped titanium dioxide (Ce3+-TiO2) hydrosols were prepared by a coprecipitation-peptization method and characterized by UV-vis transmittance spectroscopy (T%), particle size distribution (PSD), X-ray diffraction (XRD), and Brunauer-Emmett-Teller (BET) and Barret-Joyner-Halender methods (BJH), respectively. The results demonstrated that as the doped Ce3+ content increased, the crystalline size, BET surface area and transmittance decreased significantly, but the particle size increased gradually. The photocatalytic activity of Ce3+-TiO2 hydrosols was evaluated in aqueous solution for methylene blue (MB) and 2,3-dichloriphenol (2,3-DCP) degradations, and also in gaseous phase for benzene degradation. The results showed that the overall photocatalytic activity of Ce3+-TiO2 hydrosols in aqueous and gaseous phases under UVA and visible illumination was significantly higher than pure TiO2 hydrosol due to its better separation of electron-hole pairs and visible light response. Additionally, the formation of the surface complex of TiO2 and 2,3-DCP with visible light response is also contributed to the 2,3-DCP degradation, and the relevant possible reaction mechanisms were discussed with details. The kinetic data demonstrated that the Ce3+-TiO2 hydrosols with the content of Ce3+ doping between 0.5 and 1% achieved the best performance in both the aqueous and gaseous phases. This study provided the comprehensive understanding of the Ce3+-TiO2 hydrosol characteristics and reaction mechanisms, and the results indicate that these Ce3+-TiO2 hydrosols may have good potential for pollutant degradation either in aqueous phase or gaseous phase. (C) 2009 Elsevier B.V. All rights reserved.