Studies by others suggest that the reduced vitamin B-12 complex, cob(II)alamin, scavenges nitric oxide to form air-sensitive nitroxylcobalamin (NO--Cbl(III); NOCbl) in vivo. The fate of newly formed NOCbl is not known. A detailed mechanistic investigation of the oxidation of NOCbl by oxygen is presented. Only base-on NOCbl reacts with O-2, and the reaction proceeds via an associative mechanism involving a peroxynitritocob(III)alamin intermediate, CO(III)-N(O)OO-. The intermediate undergoes O-O bond homolysis and ligand isomerization to ultimately yield NO(2)Cbl and H(2)OCbl(-)/HOCbl, respectively. Ligand isomerization may potentially occur independent of O-O bond homolysis. Formation of (OH)-O-center dot and (NO2)-N-center dot intermediates from O-O bond homolysis is demonstrated using phenol and tyrosine radical traps and the characterization of small amounts of a corrinoid product with minor modifications to the corrin ring.