Peroxynitrite (O=NOO-) is formed by the reaction of cold aqueous solutions of hydrogen peroxide and nitrous acid, followed by rapid quenching with base. Reduction of this peroxy anion with sulfite (O=NOO- + SO32- --> NO2- + SO42-) at pH 12-14 is slow but is catalyzed markedly by dissolved Cu(II), which, in this medium, exists predominantly as Cu(OH)(4)(2-). Nonexponential kinetic profiles for 10 runs, carried out with [OH-] = 0.005-0.50 M, [O=NOO-] = 0.16-0.58 mM, [SO32-] = 7.5-75 mM, and [Cu-II] = 1.0-4.0 mu M, are consistent with a sequence legs 8-13 in the text) in which Cu(I) is generated from the one-electron reduction of Cu(II) by SO32-, after which Cu(I) reduces peroxynitrite to NO2 by competing protonated and nonprotonated paths. Subsequent reduction of NO2 is taken to be rapid. The proposed sequence then attributes the catalytic role of copper, in this system, to its ability to support a single-electron route to supplement the uncatalyzed path, which has been taken to entail direct oxygen atom transfer. Vanadium(V) and molybdenum(VI), which exist in this medium predominantly as the oxo anions VO43- and MoO42-, are devoid of catalytic action. The couples Fe(II,III) and Mn(II,III) are also catalytically inactive, presumably due to the very low solubility of several of the metal hydroxides at the high pH values employed.