Hydrogenation of residual oil with a viscosity of 225 centipoise at 45.4 degrees C was carried out in the presence of dispersed water soluble Ni-Mo catalyst and alumina supported Co-Mo catalyst in order to compare: their hydrotreating activities. The experiments were conducted in batch mode under a pressure of 7 MPa of H-2 and 340 degrees C using a 300 ml high pressure stirred reactor. The results revealed that presulfidation treatment of the supported Co-Mo catalyst was important. Hydrodenitrogenation (HDN) of residue over dispersed water soluble Ni-Mo was compatible with that of a commercial presulfided Co-Mo/Al2O3 catalyst. A longer reduction time was effective for reduced Co-Mo/Al2O3 catalyst in hydrodesulfurization (HDS) and HDN activities. Longer reaction time also increased the dispersed catalyst performance.