Photooxidation of dibenzothiophene in the presence of molecular oxygen and a photosensitizer like anthraquinone (AQ) proceeded in MeCN, AcOH, acetone and MeOH but not in benzene or n-octane, indicating the requirement of polar solvents. The addition of MeCN to nonpolar solvent initiated the AQ-sensitized photooxidation even when the solvent was biphasic as in an n-octane/MeCN mixture. The major oxidation product of dibenzothiophene was the corresponding sulfoxide. 4,6-Dimethyldibenzothiophene is a very unreactive sulfur compound during hydrodesulfurization but its photooxidation proceeds about 4 times faster than that of dibenzothiophene during the co-photooxidation. Sulfur content in light gas oil was reduced to below 10 ppm in the AQ-sensitized photooxidation in biphasic system consisting of light gas oil and MeCN