The selective hydrodesulfurization (HDS) of FCC gasoline is a key catalytic process for reducing sulfur content in gasoline. In addition, 5.75% of the European fuels used for transportation will have to incorporate biofuels, which implies that gasoline hydrotreatment will treat feeds containing various oxygenated compounds including by-products such as CO, CO2 or H2O. In the present work, we focus on the effect of CO partial pressure on the transformation of a model FCC gasoline composed of 2-methylthiophene (2MT) and 2,3-dimethylbut-2-ene (23DMB2N) molecules, over a alumina supported CoMoS catalyst. A negative and reversible impact of carbon monoxide on the conversion of 2MT and 23DMB2N is found. Moreover, the hydrodesulfurization (HDS) of 2MT alone is much more inhibited (30%) than hydrogenation (HYD) of 23DMB2N alone (10%). In contrast, when considering the model feed where 23DM82N and 2MT are mixed together, the loss of HDS and HYD activities is comparable whatever the CO partial pressure. Density functional theory (DFT) calculations of CO adsorption on the S- and M-edge sites of the CoMoS particles show that CO adsorption is strongly favored with respect to olefin and 2MT adsorption on both types of sites, which explains its strong inhibiting effect on HYD and HDS. A rational explanation of the different inhibiting effects of CO observed on model molecules alone and model feed is proposed in correlation with the nature of the active S- and M-edge sites present on the hexagonal CoMoS particles and involved either in HDS or in HYD reactions. (C) 2010 Elsevier B.V. All rights reserved.