The dependence of the hydrodesulphurization (HDS) activity of monometallic transition metal sulphides for thiophenic species is an electronic effect : the quality of the active site determines the rate of the rate-limiting step in the HDS process. It is shown that the strength of the interaction between the metal d electrons and sulphur 3p electrons, I, correlates well with catalytic activity. I depends strongly on the oxidation state of the transition metal. The synergistic promotion effect in Co-Mo-S and Ni-Mo-S is explained in terms of electron transfer from Co(Ni) to Mo. This corresponds to the removal of sigma metal-sulphur antibonding electrons from Co(Ni), greatly increasing their intrinsic catalytic activity. It follows that Co and Ni are the active elements in these mixed sulphides.