Bismuth vanadate (BiVO4) powder with a scheelite-monoclinic (s-m) structure was synthesized by a photoassisted sol-gel method, i.e., a modified sol-gel method including an additional procedure in which a mixed solution of BiCl3, VO(OC3H7)(3), and sodium isopropoxide was irradiated with UV light before hydrolysis. The BiVO4 powder prepared by the photoassisted sol-gel method (called P-sample) exhibited 1.3-3 times higher activity in oxygen photoevolution reaction than that prepared by the conventional nonphotoassisted sol-gel method (called N-sample), indicating that the photoassisted sol-gel method is effective for improving the photocatalytic activity of BiVO4. X-ray diffraction patterns and UV-visible absorption spectra were the same between the P- and N-samples, whereas SEM inspection showed that the P-sample was composed of well-separated small-size particles, compared with the N-sample. X-ray photoelectron spectroscopy inspection also exhibited that the P-sample had a thin surface layer containing Bi5+ and OH-. It is discussed that the high activity of the P-sample is attributed either to the small size of BiVO4 particles or a certain catalytic action of the surface layer containing Bi5+ and OH-. (c) 2005 The Electrochemical Society. All rights reserved.