The decay of peroxynitrite [O=NOO-, oxoperoxonitrate(1-)] was examined as a function of concentration (0.050-2.5 mM), temperature (5-45degreesC), and pH (2.2-10.0). Below 5degreesC and pH 7, little amounts of the decomposition products nitrite and dioxygen are formed, even when the peroxynitrite concentration is high (2.5 mM). Instead, approximately greater than or equal to90% isomerizes to nitrate. At higher pH, decomposition increases at the expense of isomerization, up to nearly 80% at pH 10.0 at 5degreesC and 90% at 45degreesC. Much less nitrite and dioxygen per peroxynitrite are formed when the peroxynitrite concentration is lower; at 50 muM and pH 10.2, less than or equal to40% decomposes. In contrast to two other reports (Pfeiffer, S.; Gorren, A. C. F.; Schmidt, K.; Werner, E. R.; Hansert, B.; Bohle, D. S.; Mayer, B, J, Biol, Chem. 1997, 272,34653470, and Coddington, J. W.; Hurst, J. K.; Lymar, S, V J. Am. Chem, Soc. 1999, 121, 2438-2443), we find that the extent of decomposition is dependent on the peroxyntrite concentration.