Using first-principles calculations. we investigate the selective adsorption of 2-methylthiophene and 2,3-dimethylbut-1-ene on the Co(Ni)MoS active edge sites. These two model molecules being relevant for the selective HDS/HydO reaction of FCC gasoline, it is shown that their relative adsorption energies at the edges must be regarded as a key chemical descriptor governing the resulting HDS/HydO selectivity. The experimentally observed higher selectivity of CoMoS is explained on the basis of the reactants adsorption selectivity and the evaluation of the active edge energies with the adsorbed reactants. The effects of the promoter content, the morphology (S-edge/M-edge) and the H2S/H-2 partial pressure on the selectivity descriptor are investigated in details. A consistent comparison with recent kinetic modeling of the HDS/HydO selectivity is provided on the basis of volcano-curves relationships. (C) 2008 Elsevier Inc. All rights reserved.