A series of Ce-TiO2 are prepared by the sol-gel process with ammonium cerium(IV) nitrate and tetra-n-butyl titanium as raw materials and characterized with XRD, TEM, DRS, and XPS. The aberrance (0.355) of anatase(101) plane of 0.1% Ce/TiO2 increases compared to that of TiO2. Cerium is found to be present as a mixture of Ce3+/4+ oxidation states, that is, the oxidation state of some cerium in Ce-TiO2 has changed after calcination. Ce/TiO2 yields a large red shift compared to TiO2. Moreover, the reflectance decreases in the visible region after cerium doped on TiO2, and the lower the reflectance the higher the activity. Cerium appears to enhance the photocatalytic activity of Ce/TiO2 by suppressing electron-hole recombination with electron trapping at Ce4+. 0.1% Ce/TiO2 shows highest activity for the degradation of aqueous suspension of formaldehyde. (c) 2005 Elsevier B.V. All rights reserved.