In connection with the specific requirements of the hydrotreating of FCC gasoline, the selectivity of hydrotreating catalysts in hydrodesulfurization with respect to the hydrogenation of olefins has to be controlled and if possible improved. The aim of this study was, on the one hand, to ascertain the role of Co and Ni on the activity of MoS2 catalysts in the activation of dihydrogen and in the hydrogenation of olefins and, on the other hand, to identify the origin of the hydrogen involved in the hydrogenation reaction. It was shown that the promotion effect of Ni on both the hydrogenation of cyclopentene (150 degrees C, fixed bed reactor) and the hydrogenation of propene (80 degrees C, recycling reactor) was higher than that of Co. This is in accordance with the fact that CoMo catalysts are more efficient in the selective hydrodesulfurization of FCC gasoline than NiMo catalysts. The hydrogenation of propene was carried out on catalyst samples, which were pre-treated with D2S at the reaction temperature (80 degrees C) then with helium at 200 degrees C in order to desorb part of it. Under these conditions it was shown that D-atoms coming from D2S were added to propene to form mainly mono-deuterated propane. No isotopic exchange between propane and D-2 was detected under these conditions. The comparison of the initial product distributions in propane and in gas phase hydrogen indicated that D-atoms issuing from D2S were added to propene directly. This was particularly clear with the NiMo/Al2O3 catalyst and also with the non-promoted catalyst. With the CoMo/Al2O3 catalyst, the activity in isotopic exchange was so high that the complete H-D scrambling between D2S and H-2 occurred even at very low hydrogenation conversion so that it was not possible to conclude. It was also found that the presence of gas phase hydrogen was necessary for the reaction to occur, which is considered further evidence in favour of the heterolytic dissociation of H-2 on sulfides. (c) 2005 Elsevier B.V. All rights reserved.