Silica/titania and zirconia/titania mixed oxides were prepared by the sol-gel technique and their photocatalytic activity for the decomposition of trichloroethylene was investigated as a function of the content of silica or zirconia. It was tried to interpret the change of photocatalytic activity in connection with the change of crystallite size of anatase phase. For both silica/titania and zirconia/titania particles, the photoactivity strongly depended on the crystallite size of anatase phase regardless of the content of silica or zirconia and the calcination temperature. On changing the crystallite size, silica/titania particles showed a different behavior in the change of the photoactivity with changing the crystallite size. The discrepancy was connected to the quantization of band structure which was identified by the blue shift in the UV-Visible absorption spectrum. On the other hand, for zirconia/titania particles the dependency of the photoactivity on the crystallite size was not affected by the content of zirconia. Finally, it was found that the photoactivity of titania-based mixed oxides prepared by the sol-gel method linearly depends on the crystallite of anatase phase. (C) 2004 Elsevier B.V. All rights reserved.