The effect of an acidic vegetable oil (AVO) on the hydrodesulfurization (HDS) reaction of an atmospheric heavy gasoil over a commercial CoMo/gamma-Al2O3 catalyst was studied. The coprocessing of the two liquid feeds was carried out in a trickle-bed bench-scale reactor at the typical HDS reaction conditions used in most refineries; namely, the reaction temperature ranged from 310 to 350 degrees C, while the total pressure was kept constant at P = 33 bar. The AVO content in the liquid feed varied from 0 to 20 wt %, and it was found that its presence inhibited the HDS reaction in all different conditions tested, which reflects an obvious decrease in the HDS reaction rate constant. Catalytic selectivity concerning the hydrodeoxygenation reaction was determined via analysis of the gaseous products. Decarboxylation cannot be distinguished from a decarbonylation mechanism because of the simultaneously occurring water-gas shift reaction. It was obvious that the overall selectivity for decarboxylation and decarbonylation reaction paths decreased with increasing AVO content in the feed and increased with the reaction temperature. During the experimental tests, the catalyst activity was measured and deactivation was found to be negligible.