Dedicated to Professor Albert Eschenmoser on the occasion of his 75th birthday Peroxynitrite (ONOO À , oxoperoxonitrate(1 À)), an isomer of nitrate that oxidizes and nitrates biomolecules, is likely to be formed in vivo from the reaction of superoxide with nitrogen monoxide. To determine whether flavonoids scavenge peroxynitrite as postulated in the literature, we studied the reactions of peroxynitrite with phenol, hydroquinone, catechol, and the flavonoid monoHER. These reactions are first-order with respect to peroxynitrous acid and zero-order with respect to the organic compounds, and proceed as fast as the isomerization of peroxynitrous acid to nitrate. In vivo, a large fraction of all peroxynitrite is likely to react with carbon dioxide to form an unstable adduct, the 1-carboxylato-2-nitrosodioxidane anion (ONOOCO À 2 ). The presence of phenolic compounds did not influence the rate of disappearance of this adduct, which was ca. 4 Â 10 2 s À1 . On the basis of these kinetic studies, it can be concluded that flavonoids are not scavengers of peroxynitrite. The products from the reaction of peroxynitrite with hydroquinone (benzene-1,4-diol) and catechol (benzene-1,2-diol) are para-benzoquinone and ortho-benzoquinone, respectively; no nitrated products were found. In a subsequent reaction, ortho-quinone reacted with nitrite, a common contaminant of peroxynitrite preparations to form 1,2-dihydroxy-4-nitrobenzene. We also investigated whether carbonyl compounds could redirect the reactivity of peroxynitrite toward nitration, as carbon dioxide does. The reaction with acetone is first-order with respect to peroxynitrite and first-order with respect to the carbonyl compound. The rate constant is 1.8 m À1 s À1 at neutral pH and 208; peroxynitrite does not react with the carbonyl compounds dimethyl acetamide, lalanylalanine, or methyl acetate. It is not likely that the carbonyl compounds or the mono-, di-, or polyphenolic compounds can scavenge peroxynitrite in vivo.