Peroxynitrite (ONOO-/ONOOH), a putative cytotoxin formed by combination of nitric oxide (NO.) and superoxide (HO2.) radicals, is decomposed catalytically by micromolar concentrations of water-soluble Fe(l I I) porphyrin complexes, including 5,10,15,20-tetrakis(2',4',6'-trimethyl-3,5-disulfonatophenyl)porphyrinatoferrate(7-), Fe(TMPS); 5,10,15,20-tetrakis(4'-sulfonatophenyl)porphyrinatoiron(3-), Fe(TPPS); and 5,10,15,20-tetrakis(N-methyl-4'-pyridyl) porphyrinatoiron(5+), Fe(TMPyP). Spectroscopic (W-visible), kinetic (stopped-flow), and product (ion chromatography) studies reveal that the catalyzed reaction is a net isomerization of peroxynitrite to nitrate (NO3-). One-electron catalyst oxidation forms an oxoFe(IV) intermediate and nitrogen dioxide, and recombination of these species is proposed to regenerate peroxynitrite or to yield nitrate. Michaelis-Menten kinetics are maintained accordingly over an initial peroxynitrite concentration range of 40-610 muM at 5.0 muM catalyst concentrations, with K-m in the range 370-620 muM and limiting turnover rates in the range of 200-600 s(-1), Control experiments indicate that nitrite is not a kinetically competent reductant toward the oxidized intermediates, thus ruling out a significant role for NO2. hydrolysis in catalyst turnover. However, ascorbic acid can intercept the catalytic intermediates, thus directing product distributions toward nitrite and accelerating catalysis to the oxidation limit. Additional mechanistic details are proposed on the basis of these and various other kinetic observations, specifically including rate effects of catalyst and peroxynitrite concentrations, solution pH, and isotopic composition.