The denitrogenation behavior of light oils, when occurring during a novel desulfurization process based on alkylation and a subsequent precipitation method, using the alkylating agents (CH3I and AgBF4), has been investigated. Denitrogenation results, obtained fur three model nitrogen compounds (aniline, indole, and carbazole) in a xylene solution, were compared with the results for three light oils, of differing nitrogen, sulfur, and aromatic concentration. The nitrogen compounds in the light oils were found to be N-methylated by reaction with the alkylating agents, such that they were removed as precipitates under moderate conditions. By use of this process, the nitrogen contents of all of the light oils were reduced successfully to less than 20% of the corresponding feed concentration, thus demonstrating that the present process is satisfactory for the simultaneous denitrogenation of light oils combined with desulfurization. Although the denitrogenation of aniline and indole compounds from the light oils proceeded effectively, carbazoles and especially those having a large carbon number of alkyl substituents were difficult to be denitrogenized by the present process. This is because the electron density on the nitrogen atom decreases with increasing carbon number of the alkyl substituents on the molecule of the carbazoles.