In a recent publication [Nauser et al. (2001) Chem. Res. Toxicol. 14, 248−350], the authors estimated a value of 14 ± 3 kcal/mol for the standard Gibbs energy of formation of ONOO^- and argued that the experimental value of 16.6 kcal/mol [Merényi, G., and Lind, J. (1998) Chem. Res. Toxicol. 11, 243−246] is in error. The lower value would suggest that the yield of free radicals during decomposition of ONOOH into nitrate is negligibly low, i.e., less than 0.5%, though within the large error limit given, the radical yield might vary between 0.003% and ca. 80%. The experimental value of 16.6 ± 0.4 kcal/mol was based on the determination of the rate constant of the forward reaction in the equilibrium ONOO^- ⇌ ^•NO and O₂^•- by use of C(NO₂)₄, an efficient scavenger of O₂^•- which yields C(NO₂)₃^-. Nauser et al. reported that addition of ^•NO has no significant effect on the rate of formation of C(NO₂)₃^-, and therefore the formation of C(NO₂)₃^- is due to a process other then reduction of C(NO₂)₄ by O₂^•-. In addition, they argued that Cu(II) nitrilotriacetate enhances the rate of peroxynitrite decomposition at pH 9.3 without reduction of Cu(II). In the present paper, we show that the formation of C(NO₂)₃^- due to the presence peroxynitrite is completely blocked upon addition of ^•NO. Furthermore, the acceleration of the rate of peroxynitrite decomposition at pH 9 in the presence of catalytic concentrations of SOD ([ONOO^-]/[SOD] > 30) results in the same rate constant as that obtained in the presence of C(NO₂)₄. These results can only be rationalized by assuming that ONOO^- homolyses into ^•NO and O₂^•- with k = 0.02 s^-1 at 25 °C. Thus, the critical experiments suggested by Nauser et al. fully support the currently accepted thermodynamics as well as the mode of decomposition of the ONOOH/ONOO^- system.