In this contribution the HDN catalytic behaviour of Co-Mo carbide catalysts and Co(Ni)-W carbide catalysts is compared in order to establish a rational effect of cobalt (or nickel) over Mo and W carbide HDN catalysts. The bimetallic and trimetallic catalysts were characterized by using elemental analysis, X-ray diffraction (XRD), infrared spectroscopy, Raman spectroscopy, thermo-gravimetric analysis and measurements of BET specific surface area. The catalytic performance was evaluated in a continuous flow reactor using hydrodenitrogenation of pyridine as model reaction. The incorporation of cobalt onto the structure Of Mo2C reached an optimal Co/Mo ratio of 0.43 (i.e. Co4Mo6Cx catalyst), whose HDN activity and stability was markedly higher than industrial catalysts (i.e. CoMoS/Al2O3 and NiMoS/Al2O3). Higher molar ratios facilitated the segregation of promoter. This was reflected in a poor catalytic stability not only on Co-Mo carbide catalysts, but also on the Co(Ni)-W carbide catalysts. The CoWCx bimetallic catalyst was more active in the steady state than Ni-containing catalysts. Two modes of pyridine adsorption may occur in the HDN reaction, the end-on mode appears to be the more favourable at low temperatures whereas the side-on mode is more favourable at higher temperatures. Further increasing reaction temperature over 400 degrees C leads to an increase in the hydrogenolysis reaction so more methane is produced, while the percentage of other hydrocarbon products decreased. (c) 2007 Elsevier B.V. All rights reserved.