Two series of bimodal CoMo/Al2O3 catalyst samples have been analyzed and characterized using porosimetry, temperature-programmed oxidation and NMR spectroscopy. The samples are withdrawn after varying times on stream of up to 120 days. The results show that coke and metals are deposited rather rapidly and that the pore volume of the catalyst at equilibrium is almost filled with coke and/or metals. With increasing time on stream, the coke in both the first- and third-stage samples becomes more hydrogen and sulfur deficient as evidenced by increasing concentration of aromatic carbon and a lowering in concentration of S associated with the coke. There is evidence that the sulfur in the coke is associated with the upper layers of coke and that nitrogen may adsorb preferentially during the initial coke laydown. Furthermore, it is shown that vanadium catalyzes the oxidation of coke. Model compound activity measurements show that the HDS activity is less sensitive to the effects of coke deposition than the HDN and hydrogenation activities. It is also shown that the initial metals deposits have a stronger effect on the loss of activity for HDS than for HDN and hydrogenation, indicating that different sites are involved in these hydrotreating reactions.