Four catalysts, Co-Mo and Ni-Mo, designed to cover a wide range of activity for aromatic ring hydrogenation and two catalysts, zeolite-Go-Mo, with different cracking activities were tested, in the hydrodesulfurization of dibenzothiophene (DBT) and 4,6-dimethyl-DBT (DMDBT) in a fixed-bed reactor at 360 degrees C and a hydrogen pressure of 5.4 MPa. Increasing the hydrogenation activity of the catalysts increased the mole ratio, alpha, of cyclohexylbenzenes to biphenyls produced from both reactants. Although alpha had little effect on the hydrodesulfurization (HDS) rate of DBT, the HDS rate of DMDBT increased significantly with alpha, approching the same level as for DBT at alpha=2. Introduction of zeolite H-ZSM-5 to the Co-Mo-Al catalyst increased the HDS rate of DBT at a lower value of alpha and decreased that of DMDBT. No cracking reactions of DBT and DMDBT were detected. A Go-Mo-Al catalyst containing HY zeolite displayed significant cracking activity of DMDBT defined by two routes : demethylation of benzenic rings and scission of the C-C bond connecting the benzenic rings. It resulted in increasing HDS rates of DMDBT by about threefold relative to the Go-Mo-Al catalyst, yielding toluene and benzene, at a ratio of about 3, as the main HDS products. About 80% of the DMDBT desulfurized with HY-Co-Mo-Al catalyst was converted through "cracking" intermediates, 90% of those intermediates being produced through the scission of the C-C bond connecting the benzenic rings.