A desulfurization process for light oil has been investigated based on chemical oxidation of sulfur-containing compounds over Ti-containing molecular sieve catalysts. Sulfur compounds, when dissolved in n-tetradecane (model light oil), were oxidized to the corresponding sulfoxides and sulfones in the presence of the catalysts and H2O2, and were removed successfully from the oil. However, by use of this basic process, the sulfur concentration of actual light oil failed to be reduced to the required deep desulfurization level (0.05 wt%). This is because alkyl-substituted sulfoxides and sulfones, produced during the process, are adsorbed on the surface of the catalyst, thus decreasing the catalytic activity. When desulfurization was carried out in the presence of polar acetonitrile solvent, the adsorption of these compounds onto the catalyst was suppressed significantly and the deep desulfurization was achieved successfully. In this process, the denitrogenation of light oil also proceeded effectively. The catalyst recovered showed no decrease in the catalytic activity and could be reused for further desulfurization and denitrogenation of light oil.