Peroxynitrite (ONOO-) is a major cytotoxic agent that has been implicated in a host of pathophysiological conditions; it is therefore important to develop therapeutic agents to detoxify this potent biological oxidant, and to understand the modes of action of these agents. Water-soluble iron porphyrins, such as 5,10,15,20-tetrakis(N-methyl-4‘-pyridyl)porphinatoiron(III) [Fe(III)TMPyP] and 5,10,15,20-tetrakis(2,4,6-trimethyl-3,5-sulfonatophenyl)porphinatoiron(III) [Fe(III)TMPS], have been shown to catalyze the efficient decomposition of ONOO- to NO3 - and NO2 - under physiological conditions. However, the mechanisms of ONOO- decomposition catalyzed by these water-soluble iron porphyrins have not yet been elucidated. We have shown that there are two different pathways operating in the catalytic decomposition of ONOO- by FeTMPyP. Fe(III)TMPyP reacts rapidly with ONOO- to produce oxoFe(IV)TMPyP and NO2 (k ≈ 5 × 107 M-1 s-1). The oxoFe(IV) porphyrin, which persisted throughout the catalytic decomposition of ONOO-, was shown to be relatively unreactive toward NO2 and NO2 -. This oxoFe(IV) porphyrin was also shown to react with ONOO- (k = 1.8 × 106 M-1 s-1), and it was this oxoFe(IV)-ONOO- reaction pathway that predominated under conditions of excess ONOO- with respect to Fe(III)TMPyP. The competition between the two pathways explains the highly nonlinear relationship observed for k cat with respect to ONOO- concentration. Fe(III)TMPyP is also known to catalyze the dismutation of the ONOO- precursor superoxide (O2 -•), and using stopped-flow spectrophotometry, the rate of Fe(III)TMPyP-catalyzed O2 -• dismutation has been determined to be 1.9 × 107 M-1 s-1 by direct measurement. A detailed mechanistic understanding of how iron porphyrins function in the catalytic decomposition of both ONOO- and O2 -• may prove essential in the exploration of the chemistry and biology of these reactive oxygen species, and in understanding the biological activity of these metalloporphyrins.