Spectral data are presented which indicate that chloroperoxidase is converted to compound II during the reaction with superoxide/hydrogen peroxide generated in the xanthine/xanthine oxidase system. Upon completion of compound II formation, compound II returns back to the native state of chloroperoxidase without formation of detectable intermediates. It is shown that chloroperoxidase acts as a superoxide scavenger but less effectively than superoxide dismutase. Addition of chloride ion enhances the rate of decay of compound II to native enzyme, showing that chloride ion is oxidized to a chlorine atom in the presence of compound II.