A series of Co/Al2O3 catalysts with various metal loadings were prepared by incipient-wetness impregnation. The reducibility Of cobalt oxide in these samples was investigated by the temperature-programmed reduction technique. The results show that for catalysts with low cobalt loadings the cobalt phase is present primarily as CoAl2O4. For catalysts having high cobalt contents, bulk Co3O4 is observed. However, some cobalt oxide-cobalt aluminate interaction exists. The rate of reduction of cobalt oxide on the support depends not only on the chemical nature of the species but also on the rate of nucleation. At high cobalt loadings, due to the presence of easy-to-reduce cobalt species, the rate of reduction is enhanced. Carbon monoxide hydrogenation activity measurements indicate that the activity is likely to be associated with the presence of easy-to-reduce species. However, the effects of particle size and pore blocking cannot be ruled out.