The inhibition effect of olefins on the hydrodesulfurization (HDS) and hydrogenation of olefins (HYDO) of a model FCC naphtha over CoMo-S/gamma-Al2O3 catalysts was studied. Particularly, the inhibition effect of linear; 1-octene, cyclic; cyclohexene, and branched; 2,4,4-trimethylpentenes (TMPs) and 2,5-dimethyl-2,4-hexadiene (2,4C(6)diene), olefins on the HDS of 2-methylthiophene was analyzed. All three kinds of olefins inhibited HDS. The degree of inhibition depended on both the type of olefin and on its concentration in the reaction feed. The inhibition effect was strongest for the cyclic olefin. In addition, the inhibition effect of the different types of olefins on their hydrogenation (HYDO) under an HDS atmosphere was also examined. Again, the cyclic olefin exerted the strongest inhibition effect on the HYDO reaction. In general, the degree of the inhibition effect on HDS and HYDO was found to decrease thanks to steric hindrance effects. (C) 2015 Elsevier Ltd. All rights reserved.