In neutral bicarbonate-containing solutions, the predominant pathway for peroxynitrite decomposition was CO2-catalyzed formation of nitrate ion; that is, CO2 was regenerated in the process. When nitrite ion was present, HCO3- also formed during ONO2- decomposition. The data could be reproduced by a kinetic model wherein a reactive intermediate formed from ONO2-and CO2 oxidizes NO2- to . NO2, which subsequently hydrolyzes in a bimolecular reaction to form NO3- and regenerate NO2-. Carbon dioxide catalyzed one-electron oxidations of Fe(CN)(6)(4-), Mo(CN)(8)(4-), Os(bpy)(3)(2+), Fe(bpy)(3)(2+), Ru(bpy)(2+), and I- were also observed, establishing that the reactive intermediate is capable of oxidizing compounds whose reduction potentials are as great as 1.3 V. The maximal product yields, measured for reactions with NO2-, Fe(CN)(6)(4-), and Mo(CN)(8)(4-), were all similar to 35% of the added peroxynitrite, suggesting that they were oxidized by a common intermediate. On the basis of the reactivity characteristics revealed, the intermediate is suggested to be carbonate radical formed by homolytic decomposition of an ONO2CO2- adduct.