Peroxynitrite (O=N-O-O-) is formed by the reaction, at -2 degrees C, of nitrous acid and hydrogen peroxide, followed by rapid quenching with excess cold base. Unlike its conjugate acid (peroxynitrous acid, pK(a) = 6.5), which decomposes quickly, the anion can be preserved for several weeks at high pH at -18 degrees C. Solutions of ONOO- rapidly oxidize solutions of H3AsO3, antimony(III) tartrate, and HSO3- (at pH 5-13) and Sn(II) (at pH 10-13) in reactions exhibiting 1:1 stoichiometry. Oxidation of hypophosphite is much slower and that of phosphite is nondetectable. Kinetic acidity patterns for oxidations of As(III), Sb(III), and S(IV) are each in accord with reactions proceeding via three paths featuring different levels of protonation. For H3AsO3 and antimony(III) tartrate, reaction involving the monodeprotonated reductant (e.g., H2AsO3-) and ONOOH (OxH(+) + Red) predominates whereas, for HSO3-, reaction is most rapid when both redox partners are protonated (OxH(+) + RedH(+)). All transformations in this study are taken to be ore-transfer reactions proceeding through O-bridged precursor complexes. The observed reactivity series, Sn-II > Sb-III > As-III > S-IV much greater than P-I > P-III, is not directly related to the formal potentials of these reducing centers but appears largely to reflect the relative accessibilities of the electron-rich sites of the reductants, with the substitution-inert anions H2PO2- and HPO32- being particularly unreactive. The unexpectedly high rates associated with the protonated form of S(IV) may be attributed to the decrease in coordination number of sulfur and the attendant improved availability of the reducing site as the HSO3- anion is converted, via protonation, to SO2, which is recognized to be the preponderant S(IV) species in acidic aqueous solutions.