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 to compare the reactivities in hydrogenation and in isomerization of various olefins on Mo, CoMo and NiMo catalysts in order to evaluate the promoter effect in both reactions. It was shown that the promotion effect on the hydrogenation of olefins (150 degrees C, fixed-bed reactor) was depending on their structure. However, the promotion effect of Ni was higher than that of Co whatever the olefin. This is in accordance with the fact that CoMo catalysts are more efficient in the selective hydrodesulfurization of FCC gasoline than NiMo catalysts. The presence of cobalt favours also the skeletal isomerization of the olefins. (C) 2008 Elsevier B.V. All rights reserved.