It is demonstrated that highly dispersed molybdenum sulfide catalysts are prepared by using Mo(CO)6 adsorbed on the support. The structure of ultradispersed molybdenum sulfide is suggested by EXAFS techniques to be different from that of MoS2. The TOF of the thiophene HDS is almost invariant with the dispersion of molybdenum sulfide at a high molybdenum dispersion, whereas the TOF of the hydrogenation greatly increases with increasing dispersion. On the basis of the HDS activity and optimum Co/Mo ratio of Co-Mo/Al2O3 catalysts having a variety of molybdenum dispersion fabricated by using carbonyl complexes, it is demonstrated that highyl active HDS catalysts are prepared by highly dispersing molybdenum sulfide phases. Edge decorations of molybdenum sulfides by cobalt are suggested for the synergy generation between cobalt and molybdenum sulfides.