Deep hydrodesulfurization of polyaromatic sulfur-containing compounds (PASC) in light oil was carried out by using Co-MO/Al2O3 under experimental conditions representative of industrial practice. PASC were determined by gas chromatography-atomic emission detection (GC-AED) and gas chromatography-mass spectroscopy (GC-MS). Alkyl-substituted dibenzothiophenes among sulfur-containing compounds in light oil were most difficult to desulfurize. Dialkyl-substituted dibenzothiophenes, especially 4,6-dimethyldibenzothiophene, remained until the final stage of the reaction (390-degrees-C) while alkylbenzothiophenes were completely desulfurized at 350-degrees-C.