Oxidation of the methyl viologen radical, MV.+, by the peroxynitrite anion, ONOO-, occurs through an indirect pathway, which includes reversible homolytic dissociation of ONOO- into a pair of the NO. and O-2(.-) radicals (rate-determining step, k(h) = 0.017 +/- 0.001 s(-1)) followed by rapid oxidation of MV.+ by O-2(.+) with k(so) = (3.5 +/- 0.2) x 10(9) M-1 s(-1) at zero ionic strength. Direct oxidation of MV.+ radical through a bimolecular reaction with ONOO- is immeasurably slow (k < 1 x 10(3) M-1 s(-1)). The MV.+ radical does not rapidly reduce NO., suggesting that the one-electron reduction potential of the latter is significantly below -0.45 V (NHE), consistent with the recently reported reevaluations of NO. reduction energetics. The equilibrium constant K-h = (3.6 +/- 0.3) x 10(-12) M for the dissociation of ONOO- into NO. and O-2(.-) is determined from the measurements of rate constants for the forward and reverse (recombination of NO. and O-2(.-), k(-h) = (4.8 +/- 0.3) x 101 M-1 s-1) reactions. The free energies of formation in aqueous solution Delta(f)Gdegrees(ONOO-) = 68.5 +/- 1.4 kJ/mol and Delta(f)Gdegrees(ONOOH) = 30.8 +/- 1.5 kJ/mol are derived from these data. The last value is fully consistent with the major role of the ONOOH homolytic dissociation into a pair of the NO2. and OH. radicals in oxidations by peroxynitrite. Collectively, these results help to resolve disputes in the literature over the values for k(h), k(-h), K-h, Delta(f)Gdegrees(ONOO-), and Delta(f)Gdegrees(ONOOH); measurements of all these quantities in this work disagree with their recent reevaluations, but largely agree with earlier reports.