Hydrodesulfurization (HDS) of a straight run light gas oil (SRLGO, Sulfur: 1.50wt%) and ultra-deep HDS of a hydrotreated light gas oil (HDLGO, Sulfur: 0.045wt%) were carried out over a commercial Co-Mo/Al₂O₃ catalyst presulfided with a novel sulfiding agent, bis-(1, 1, 3, 3-tetramethylbutyl)-polysulfide (CS-40). The HDS reaction conditions were: Temperature 310-390°C, total pressure 3.0MPa, LHSV 2.0-4.0h^-1, Gas/Oil ratio 125 Nm³/m³. The catalyst presulfided in situ with CS-40 showed comparable activity to the corresponding catalyst presulfided with dimethyldisulfide (DMDS), and higher activity than that presulfided with H₂S in HDS of SRLGO. Catalyst presulfided ex situ with CS-40 plus sulfur showed significantly enhanced activity for HDS of alkyl-substituted dibenzothiophenes (DBTs). No significant changes in apparent activation energies for HDS of DBT were observed. In contrast, the catalyst ex situ sulfided with CS-40 plus sulfur had smaller activation energies for HDS of 4, 6-dimethyldibenzothiophene (4, 6-DMDBT). The ex situ sulfiding method using CS-40 plus sulfur may enhance the formation of active sites for hydrogenation and results in increased activity for indirect desulfurization of alkyl-substituted DBTs via hydrogenation of the aromatic ring of DBTs to form cyclohexylbenzenes.