The decay of ONOOH near neutral pH has been examined as a function of isomerization to H(+) and NO3(-), and decomposition to NO2(-) and O2via O2NOO(-). We find that in phosphate buffer k(isomerization) = 1.11 ± 0.01 s(-1) and k(disproportionation) = (1.3 ± 0.1) × 10(3) M(-1) s(-1) at 25 °C and I = 0.2 M. In the presence of 0.1 M tris(hydroxymethyl)aminomethane (Tris), the decay proceeds more rapidly: k(disproportionation) = 9 × 10(3) M(-1) s(-1). The measured first half-life of the absorbance of peroxynitrite correlates with [Tris]0·([ONOO(-)]0 + [ONOOH]0)(2), where the subscript 0 indicates initial concentrations; if this function exceeds 6.3 × 10(-12) M(3), then Tris significantly accelerates the decomposition of peroxynitrite.