Stable nitroxide radicals were reported to act as SOD mimics and catalyze the dismutation of O2-. through two different catalytic pathways including reductive and oxidative reaction mechanisms (Samuni, A., Krishna, C. M., Riesz, P., Finkelstein, E. & Russo, A. (1988) J. Biol Chem. 263, 17921-17924). Recent studies directly monitoring O2-. and employing kinetics analysis did not reveal SOD activity of nitroxides (Weiss, R. H., Flickinger, A. G., Rivers, W. J., Hardy, M. M., Aston, K. W., Ryan, U. S. & Riley, D. P. (1993) J. Biol. Chem. 268, 23049-23054). Such discrepancy may result in cases where distinction of stoichiometric scavengers from catalytic detoxifiers of O2-. is not readily feasible. Nitroxides are effective antioxidants that protect against oxidative injury in various pathological processes. The distinction of their SOD mimic activity from O2-. scavenging was established by examining the validity of direct and indirect methods employed to assay SOD-like catalytic activity. Kinetics analysis along with direct EPR monitoring were used to study the mechanism underlying nitroxide reactions with O2-.. The nitroxide EPR signal decayed in the presence of NADH but otherwise did not decrease with time, thus substantiating its catalytic role in O2-. dismutation. The catalytic rate constants for O2-., dismutation, determined for the nitroxides tested, were found to increase with [H+], indicating that .OOH rather than O2-. is oxidizing the nitroxide. The results demonstrate the limitations associated with direct kinetics analysis in evaluating SOD mimic activity, underscoring the need for independent assays for valid discrimination of SOD mimics from stoichiometric scavengers of O2-..