Superoxide ion (O2-) is formed in the aqueous phase during decomposition of hydrogen peroxide over metal oxides supported on alumina. The kinetics of the decomposition and superoxide ion formation have been studied. The same kinetic equations can be applied as previously reported for the decomposition over metals supported on alumina. The kinetic results are explained based on Weiss's mechanism. A correlation between the catalytic activities of metal oxides for the decomposition and the oxidation potential of metal cations are accounted for by the mechanism.