The simultaneous removal of sulfur- and nitrogen-containing compounds from vacuum gas oil (VGO), based on alkylation and a subsequent precipitation method using alkylating agents (CH3I and AgBF4), has been investigated. Desulfurization and denitrogenation reactivities for various compounds in VGO have been studied by means of field ionization-mass spectrometry (FI-MS) and gas chromatography-atomic emission detection (GC-AED), respectively. The sulfur and nitrogen compounds in the VGO were found to be methylated by the addition of the alkylating agents under moderate conditions and were removed successfully as the precipitates of the corresponding S-methylsulfonium and N,N-dimethylcarbazolium tetrafluoroborates. By this means, the sulfur and nitrogen concentrations of the VGO were reduced simultaneously to less than 0.1% and 7.0% of the corresponding feed values, in the presence of a 20-fold molar excess of CH3I and a 10-fold molar excess of AgBF4, respectively, based on the initial sulfur concentration of the feed VGO. The FI-MS and GC-AED analyses revealed that the tetrahydrodibenzothiophenes and carbazoles, especially those having a large carbon number of alkyl substituents, are the most difficult compounds to be removed by the process.