The promoter effect of Co or Ni on the hydrodesulfurization (HDS) activity of Mo/alumina was studied by using dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (46DMDBT) as reactants. The reaction was carried out at 340 degrees C under a 4 MPa total pressure in a fixed-bed microreactor. On the Mo/alumina catalyst, both reactants had similar reactivities, 46DMDBT being even slightly more reactive than DBT. However, as generally observed, on the CoMo/alumina and NiMo/alumina catalysts, DBT was much more reactive (5 to 6 times) than 46DMDBT. This was mainly because of a tremendous enhancement of the rate of the so-called "direct desulfurization" (DDS) pathway of the HDS of DST, whereas for 46DMDBT this effect was much more limited. It was therefore concluded that the main effect of the promoter on the HDS of DBT-type molecules was to increase the rate of the C-S bond cleavage provided this reaction was not hindered by steric constraints. This effect was attributed to an enhancement by the promoter of the basicity of certain sulfur anions in its vicinity. It was also shown that the lower reactivity of 46DMDBT compared to that of DBT measured on the promoted catalysts could not be attributed to differences in the adsorption strength of the reactants, Assuming that CS bond cleavage occurred through a B-elimination process, several other explanations for the low reactivity of 46DMDBT were proposed and discussed: (a) steric hindrance of the adsorption of the dihydrointermediates by the methyl groups; (b) steric hindrance by the methyl groups of the C-S bond cleavage; (c) the fact that only one H atom is available for the C-S bond cleavage; (d) an effect of the methyl group on the acidity of the H atom involved in the elimination step. Proposals were also made concerning the catalytic centers involved in the hydrogenation steps and in the C-S bond cleavage steps,