Activation volumes (DeltaV(double dagger)) have been determined for several reactions of peroxynitrite using the stopped-flow technique. Spontaneous decomposition of ONOOH to NO3- in 0.15 M phosphate, pH 4.5, gave DeltaV(double dagger) = 6.0 +/- 0.7 and 14 +/- 1.0 cm(3) mol(-1) in the presence of 53 muM and 5 mM nitrite ion, respectively. One-electron oxidations of Mo(CN)(8)(4-) and Fe(CN)(6)(4-), which are first order in peroxynitrite and zero order in metal complex, gave DeltaV(double dagger) = 10 +/- 1 and 11 +/- 1 cm(3) mol(-1), respectively, at pH 7.2. The limiting yields of oxidized metal complex were Found to decrease from 61 to 30% of the initially added peroxynitrite for Mo(CN)(8)(3-) and from 78 to 47% for Fe(CN)(6)(3-) when the pressure was increased from 0.1 to 140 MPa. The bimolecular reaction between CO2 and ONOO- was determined by monitoring the oxidation of Fe(CN)(6)(4-) by peroxynitrite in bicarbonate-containing 0.15 M phosphate, pH 7.2, for which DeltaV(double dagger) = -22 +/- cm(3) mol(-1). The Fe(CN)(6)(3-) yield decreased by similar to 20% upon increasing the pressure from atmospheric to 80 MPa. Oxidation of Ni(cyclam)(2+) by peroxynitrite, which is first order in each reactant, was characterized by DeltaV(double dagger) = -7.1 +/- 2 cm(3) mol(-1), and the thermal activation parameters DeltaH(double dagger) = 4.2 +/- 0.1 kcal mol(-1) and DeltaS(double dagger) = -24 +/- 1 cal mol(-1) K-1 in 0.15 M phosphate, pH 7.2. These results are discussed within the context of the radical cage hypothesis for peroxynitrite reactivity.