Catalysts of cobalt oxide loaded on smectic clays having high surface areas were prepared. Smectic clays used were montmorillonite, saponite, porous saponite, hectorite and stevensite. The catalysts were tested for hydrodesulfurization (HDS) activity with thiophene in a pulse flow reactor at different temperatures. Go-porous saponite catalyst prepared by an ion-exchange method in the series including CoMo-Al2O3 shows the highest thiophene HDS activity so far studied. The catalytic properties of the selected catalysts were characterized by temperature-programed-reduction (TPR) and -sulfiding (TPS) together with electron spin resonance (ESR). The HDS performance with thiophene is closely dependent upon the reduction temperature of cobalt oxide obtained by TPR for Co-smectite catalysts. The TPS measurements indicate that small particles of cobalt oxide are highly dispersed in the smectic layer by means of ion-exchange method. It is suggested that the properties such as sensitivity toward H2S and catalyst color of cobalt oxide on Go-porous saponite by ion-exchange method are different from those of Go-porous saponite by impregnation method. Reactivity of loaded cobalt oxide with H2S is strongly dependent upon the sulfiding paths.