Presence of the superoxide ion (O2-) in the aqueous phase during the decomposition of hydrogen peroxide over supported metals has been revealed by quenching the reacting solution and measuring the ESR of the quenched ice. Observation of the superoxide ion in an aqueous phase gives direct evidence for the involvement of free-radical intermediates during peroxide decomposition by solid catalysts. The kinetics of hydrogen peroxide decomposition and of superoxide formation were studied. The rate of superoxide formation is expressed by d[O2-]/d[W/F] = k2[H2O2]t - k3[O2-], where [H2O2]t is the total analytical concentration of hydrogen peroxide, W is the amount of catalyst, and F is the flow rate of the solution. A reaction mechanism for the formation of the superoxide ion has been proposed based on the Haber-Weiss mechanism. It was also found that the superoxide ion trapped in ice decayed with second-order kinetics above -70°C with an activation energy of 50 kJ/mol.