A novel desulfurization process for light oils has been investigated, based on the precipitation of S-alkylsulfonium salts produced by the reaction of sulfur-containing compounds with alkylating agents (CH3I and AgBF4). Dibenzothiophenes (DBTs) and benzothiophenes (BTs), present in the light oils, were methylated by the addition of the alkylating agents, in the presence of dichrolomethane, to be removed as precipitates under moderate conditions. A semiempirical molecular orbital (MO) calculation shows that the desulfurization reactivity for both DBTs and BTs increases with increasing electron density on the sulfur atom, lying parallel to the plane of the molecules, and that the desulfurization activity for BTs decreases with electron density (bond order) on the unsaturated bond in the thiophene ring. The desulfurization of light oil, containing a high amount of aromatic hydrocarbons, was relatively difficult, because the methylation of aromatics occurs competitively with the S-methylation reaction. Desulfurization was, however, accelerated by an increase in the quantity of the alkylating agents added, such that the sulfur content of the light oils was decreased successfully to less than 0.005 wt %. The resulting light oil products confirm that the proposed process is satisfactory for application to the deep desulfurization of light oils. The desulfurization reactivity of the individual sulfur-containing compounds in light oils was found to depend on the aromatic content of the light oils. This is because the S-methylsulfonium salts are converted to parent sulfur-containing compounds by reaction with aromatic hydrocarbons.