Peroxynitrite ion (ONO₂^-) reacted rapidly with CO₂ to form a short-lived intermediate provisionally identified as the ONO₂CO₂^- adduct. This adduct was more reactive in tyrosine oxidation than ONO₂^- itself and produced 3-nitrotyrosine and 3,3‘-dityrosine as the major oxidation products. With tyrosine in excess, the rate of 3-nitrotyrosine formation was independent of the tyrosine concentration and was determined by the rate of formation of the ONO₂CO₂^- adduct. The overall yield of oxidation products was also independent of the concentration of tyrosine and medium acidity; approximately 19% of the added ONO₂^- was converted to products under all reaction conditions. However, the 3-nitrotyrosine/3,3‘-dityrosine product ratio depended upon the pH, tyrosine concentration, and absolute reaction rate. These data are in quantitative agreement with a reaction mechanism in which the one-electron oxidation of tyrosine by ONO₂CO₂^- generates tyrosyl and NO₂ radicals as intermediary species, but are inconsistent with mechanisms that invoke direct electrophilic attack on the tyrosine aromatic ring by the adduct. Based upon its reactivity characteristics, ONO₂CO₂^- has a lifetime shorter than 3 ms and a redox potential in excess of 1 V, and oxidizes tyrosine with a bimolecular rate constant greater than 2 × 10⁵ M^-1 s^-1. In comparison, in CO₂-free solutions, oxidation of tyrosine by peroxynitrite was much slower and gave significantly lower yields (∼8%) of the same products. When tyrosine was the limiting reactant, 3,5-dinitrotyrosine was found among the reaction products of the CO₂-catalyzed reaction, but this compound was not detected in the uncatalyzed reaction.