A novel deep desulfurization process of light oil, effected by a combination of photochemical reaction and organic two-phase liquid-liquid extraction, has been investigated. The process is comprised of two stages. The first consists of the transfer of the sulfur-containing compounds from the Light oil to an aqueous-soluble polar solvent. This is then followed by the photooxidation and photodecomposition of the sulfur-containing compounds in the solvent by UV irradiation, using a high-pressure mercury lamp. The operations are carried out under conditions of room temperature and atmospheric pressure. Acetonitrile was found to be the most suitable polar solvent for the process. Tn acetonitrile, dibenzothiophene (DBT) is converted to DBT 5-monoxide and then to DBT 5,5-dioxide, dibenz[c,e][1,2]oxathiin 6-oxide, and aromatic sulfonate or sulfinate anion by the UV irradiation. These products are highly polarized and are therefore not distributed into the nonpolar light oil phase. An adverse effect of naphthalene, in depressing the photodecomposition of DBT, is actually reduced by the use of an acetonitrile solution. The desulfurization yield for commercial light oil (CLO) and for straight-run light gas oil (LGO) is drastically improved by the present process, the sulfur content of CLO being reduced from 0.2 to 0.05 wt % by 2 h of irradiation ana that of LGO from 1.4 to 0.05 wt % by 10 h of irradiation. The separation of the coextracted aromatics from the acetonitrile was carried out successfully by using light paraffinic hydrocarbon stripping agents.