The self-decomposition of peroxynitrite in strongly basic solution, principally to O-2 and nitrite (2O=N-O-O--->O-2 + 2NO(2)(-)), is catalyzed markedly by the sulfito-substituted cobalt(III) complexes [CO(NH3)(5)(SO3)](+) and cis-[Co(NH3)(4)(SO3)(2)](-) but not by cobalt compounds devoid of Ligated sulfite. Catalysis does not entail recycling of cobalt between di-and tripositive states. As a result of trans-labilizing action of bound sulfite, the bis(sulfito) catalyst is taken to be converted to the corresponding cis-(H2O)(2) complex, which, in the basic medium employed, is deprotonated to a mixture of mono-and dihydroxo complexes (both reactive), thus accounting for the two paths indicated by kinetic data taken in the presence of this catalyst. It is proposed that catalytic action is initiated when peroxynitrite replaces coordinated water at the Co(III) site, that the O-O bond undergoes homolysis by a Fenton-like act, except that the electron lost from the catalyst arises from a ligating sulfito group rather than Co(III), and that a second peroxynitrite anion rapidly undergoes two successive le(-) oxidations to yield O-2 and NO2-. The singular catalytic effectiveness of sulfito-bound cobalt(III) is attributed to a combination of the trans-effect of this ligand and its ability to undergo single-electron oxidation.