Substituted spinel oxides such as ZnxCo3-xO4 (x=0-1), AlxCo3-xO4 (x=0-2.5) and FexCo3-xO4 (x=0-2.5) were synthesized to clarify the relation between the surface cobalt species and the oxidation activity/selectivity. These cobalt oxides of the spinel type contain various cobalt species, such as cobalt(II) in tetrahedral sites (Co-Td(2+)), cobalt(III) in octahedral sites (Co-Oh(3+)) and cobalt(II) in octahedral sites (Co-Oh(2+)). From XRD it was concluded that the crystal structure of ZnxCo3-xO4, AlxCo3-xO4 and FexCo3-xO4 was that of the spinel oxide in the range of x=0-1,0-2,0-2, respectively. Co-Td(2+) and Co-Oh(3+) in the crystals are replaced by Zn2+. and 2+ Al3+, respectively while iron(III) replaces Co-Oh(3+) in the range of x from zero to one, and Co-Td(2+) from one to two. Similar replacement of cations was confirmed at the surface from XPS of Co 2p level. Oxidation of CO/H-2 mixed gas was conducted at 50-450 degrees C on the substituted cobalt spinel oxides. The products were water and carbon dioxide. The substituted cobalt oxide showed high oxidation activity and CO oxidation selectivity only when Co-Oh(3+) was observed on the surface, while it showed quite low activity and selectivity when little Co-Oh(3+) was observed. It is concluded that the main factor which determined the activity and selectivity for CO oxidation in the presence of H-2 is Co-Oh(3+) on the surface of the catalyst. On the other hand, the lack of a quantitative correlation between the oxidation rate and the quantity of Co-Oh(3+) in the range of x=0-1.5 suggests that other secondary factors also contribute to the activity.