Cu-catalyzed oxidation of ascorbate has been studied in the absence and the presence of superoxide dismutase, catalase, mannitol, glycerol, ethanol, formate, and thiourea. None of these agents except thiourea inhibited the reaction. Therefore, the role of the Haber-Weiss reaction in the ascorbate oxidation could not be demonstrated. Electron spin resonance studies demonstrated that the preventive effect of the thiol is primarily due to the chelation of the reduced copper ions with the sulphur atom. The oxidation was also prevented by the chelation of copper with physiological levels of bovine serum albumin. These observations are consistent with the concept that a metal-oxygen complex is perhaps directly involved in the oxidative process. Measurements of the peroxide produced during oxidation indicated that significant amounts of this compound accumulates only at lower levels of ascorbate and in the absence of a protein or other chelating agents. At higher ascorbate levels no peroxide accumulation takes place. These results are, thus, useful in predicting the conditions under which the nutrient may act as a pro-oxidant or as an anti-oxidant. The observations suggest that under normal conditions low levels of ascorbate may act as a pro-oxidant through H2O2 production if the system has transition metal ions devoid of chelating agents. At higher concentrations ascorbate acts predominantly as an antioxidant.