Skeletal isomerization of 4,6-dimethyldibenzothiophene (4,6-DMDBT) was performed at 270 degrees C for 1 h at a hydrogen pressure of 2.5 MPa using a Ni-supported Y-type zeolite catalyst. Alkyldibenzothiophenes (alkyl-DBTs), which were produced by methylmigration and transalkylation, were further desulfurized at 255-300 degrees C for 2-120 min at a hydrogen pressure of 2.5 MPa using a CoMo/Al2O3 catalyst. The produced alkyl-DBTs, which carried 1-3 methyl groups, could be classified into three groups on the basis of activation energy for desulfurization. Group I included 2,8-DMDBT and C-1-DBT, which carried no methyl groups at either the 4- or 6-position. Group III included unreacted 4,6-DMDBT, and group II contained other alkyl-DBTs with 1-3 methyl groups. No alkyl-DBTs with 4 methyl groups were found in the products of 4,6-DMDBT. The steric hindrance of the methyl groups on 4,6-DMDBT was relieved by methylmigration and transalkylation, and, as a result, the desulfurization activation energy, 37.2 kcal/mol, for 4,6-DMDBT was decreased to 24-33 kcal/mol for alkyl-DBTs in group II and to 23 kcal/mol for alkyl-DBTs in group I. Desulfurization reactivity of the alkyl-DBTs was also examined on the basis of molecular orbital calculation using the WinMOPAC program. The cross sectional area of the Son orbital on the sulfur atom, which was in contact with the catalyst surface, was 0.12 nm(2) for 4,6-DMDBT, 0.70 nm(2) for 4-MDBT, and 1.27 nm(2) for DBT. Thus the desulfurization reactivity increases with increasing overlapping area of the Son orbital with the catalyst active site. A wider cross sectional area was achieved by migration of methyl groups using the zeolite catalyst.