The carbides and nitrides of Mo and W can adsorb and activate hydrogen. The effects of particle size and surface area on the total amount of adsorbed hydrogen differ from those observed for transition metal sulfides. In the former case, this amount increases with increasing particle size and/or decreasing surface area. This is consistent with the involvement of the sub-surface regions of the crystallites during hydrogen adsorption. The activity for hydrogenation, hydrodesulfurization and hydrodenitrogenation exhibits similar trends. The high HDN activity of the Mo(W) carbide and nitride based catalysts has been noted. These catalysts are stable under typical hydroprocessing conditions although a partial sulfidation of their surface during HDS cannot be avoided.