Abstract The rate constant, kS, for the reaction of scavengers, obtained from a competitive reaction system in which KO2 was used as a superoxide source, with superoxide has been reevaluated in electron spin resonance (ESR) spin-trapping studies. In consequence, it has been found that the local pH in a reaction field of our KO2 system was 12. As a result, the rate constants for the reaction of xanthine oxidase (XOD) with superoxide at pH 7.8 and 12 were established to be 2.1 × 106 and 8.0 × 105 M−1 s−1, respectively. On the basis of these values, discrepancies between reports in the literature have been examined, and it has been found that systematic errors appearing in values of the rate constant, kT, for the reaction of spin-trapping agents with superoxide due to variations in determination method had originated from omission of the superoxide-scavenging activity of XOD and that the errors could be explained almost completely quantitatively. For this reason, the kT values latterly reported by Allouch et al. are inferior to those formerly reported by Finkelstein et al. as reference values when the kS values are determined by the kinetic competition using these kT values.