A mechanism for the transformation of alkyl-dibenzothiophenes by hydrodesulfurization (HDS) on a NiMo catalyst is proposed. 4-Methyl-dibenzothiophene (4-MDBT) (1) has been chosen as a model for this study because this non-symmetrical molecule leads to the formation of a great variety of intermediates. Thanks to the synthesis of these compounds, a detailed study of the reaction course by the observation of the relative concentrations of intermediate products was realized. The results are consistent with adsorption of the molecules via the aromatic system, parallel to the catalyst surface, as proposed previously. The formation of a dihydrogenated intermediate is followed by successive hydrogenation or elimination steps. The rates of these reactions are dependent on the location of the methyl substituent and hence on the generated sterical hindrance. The relative amount observed in each compound is in total accordance with this mechanistic proposal.