Inhibition of straight run gas oil (SRGO) hydrodesulfurization (HDS) in the presence of rapeseed oil (RSO) and carbon monoxide was studied over CoMo/Al2O3 and NiMo/Al2O3 catalyst at 340 degrees C, 4.0 MPa, LHSV - 2 (1.5) h(-1) and H-2/feed ratio - 600 Nm(3)/m(3). It was found that both RSO and CO addition have only a minor effect on the behavior of NiMo/Al2O3 catalyst. The appreciable increase of the sulfur content in the products was observed after RSO or CO addition when CoMo/Al2O3 was used for HDS of the RSO-SRGO blends. It was found, that the HDS rate constant correlates with the CO content in the outlet gas irrespective of the source of CO: addition to the inlet gas or production from rapeseed oil HDO. The effect of CO on the selectivity of HDO/HDeCO(x) routes is also observed: the ratio C-18/C-17 is increased with the rising of CO concentration in the feed. The obtained results confirm the major role of CO as the inhibitor of HDS reactions in the presence in the blends with RSO. We showed that using of sulfide un-promoted Mo/Al2O3 catalyst in the front layer for the RSO conversion diminished the COx formation and allowed to use the sulfide CoMo/Al2O3 catalyst in the second layer without significant decrease of HDS activity.