A desulfurization process for light oils, based on photocatalytic oxidation of sulfur-containing compounds using TiO2 and an oil/acetonitrile two-phase extraction, has been investigated. Dibenzothiophenes (DBTs), when dissolved in acetonitrile, are oxidized by photoirradiation at wavelengths lambda > 320 nm in the presence of TiO2, to form the corresponding sulfoxides and sulfones, which do not distribute into the non-polar light oil owing to their high polarity. When light oil and acetonitrile are mixed and photoirradiated in the presence of TiO2, the sulfur compounds are extracted successively and photooxidized in acetonitrile. By this means, deep desulfurization is achieved: the sulfur content of light oil being reduced from 0.18 to less than 0.05 wt%. The present process, however, has a difficulty in the desulfurization of high-aromatic-content light oils.