Peroxynitrite is formed when (NO)-N-. is added to oxygenated solutions of hydrogen-peroxide. The formation rates and the yields of peroxynitrite were determined using the stopped-flow technique at pH 7.5-11.7. The stoichiometry of this process has been determined, and is given by 4(.)NO + O-2 + 2H(2)O(2) --> 2ONOO(-) + 2NO(2)(-) + 4H(+). Kinetic studies show that the rate law of the nitrosation process is given by -d[O-2]/dt = k(1)[(NO)-N-.](2)[O-2] with k(1) = (2.4 +/- 0.3) x 10(6) M(-2) s(-1). The kinetic results are identical to those obtained for the autoxidation of (NO)-N-., indicating that the rate determining step of the nitrosation process is the formation of ONOONO (or ONONO2 or O2NNO2), which is the precursor of (NO2)-N-. and of N2O3. The stoichiometry of the nitrosation process suggests that (NO2)-N-. and/or N2O3 are the reactive species. Competitive kinetic studies demonstrate that the yield of peroxynitrite is independent of (NO)-N-. concentrations, indicating that the reactive intermediate is N2O3. The rate constant of the reaction of N2O3 with HO2- was determined to be (1.0 +/- 0.6) x 10(9) M(-1) s(-1), whereas that with H2O2 is at least 4 orders of magnitude lower. The nitrosation of H2O2 by (NO)-N-. takes place only in the presence of oxygen, and under the conditions of this study, we found no evidence for the direct nitrosation of H2O2 by (NO)-N-..