Zeolite-supported highly dispersed Co sulfide clusters are synthesized using Co(CO)(3)(NO) as a precursor. The amount of Co in CoSx/zeolite anchored by a CVD technique increases as the Al/Si ratio of the zeolite increases, whereas the activity per Co atom decreases for thiophene hydrodesulfurization (HDS). When an Na-exchanged USY zeolite is used, the Co sulfide catalyst shows a much higher HDS activity than a conventional Co-Mo/Al2O3 catalyst. It is considered from XAFS and NO adsorption techniques that the high HDS activity of CoSx/USY-Na is due to an extremely high dispersion of Co sulfide clusters.