Desulfurization of vacuum gas oil (VGO) has been investigated based on chemical oxidation of sulfur-containing compounds using H2O2 and acetic acid (AcOH), followed by extraction of the oxidized compounds from the resulting VGO using an aqueous acetonitrile solution. The desulfurization behavior of VGO was compared with that of light oil feedstocks, and the reactivities of the individual sulfur compounds in VGO were studied by field ionization-mass spectrometry (FI-MS) analysis. By use of the desulfurization process, the sulfur content of the VGO was decreased successfully to less than 11% of the corresponding feed values, and the desulfurization yield of the VGO was significantly higher than that of light oils. This is because polyaromatic thiophenes in the VGO that possess at least one naphthenic ring adjacent to the thiophenic ring have higher electron density on the sulfur atom than the sulfur compounds contained in light oils, and are oxidized more effectively.