A discrete peroxynitrite-copper(ii) complex, [(TMG(3)tren)Cu-II((-OONO)-O-=)](+) (3), has been generated in solution (ESI-MS, m/z = 565.15; tetragonal EPR) by reacting center dot NO(g) with superoxo complex [(TMG(3)tren)Cu-II(O-2(center dot-))](+) (2). Complex 3 undergoes a thermal transformation to give Cull-nitrite complex [(TMG3tren) Cu-II(-ONO)](+) (4) (X-ray) along with ca. 0.5 molar equiv dioxygen. A DFT calculation derived structure with cyclic bidentate k(2)-O,O '-OONO bound peroxynitrite moiety and d(x)2-(y)2 ground state is proposed. Experiments using 1802 suggest that the adjacent peroxo oxygen atoms in 3 are derived from molecular oxygen. Further, 1802 containing 3 undergoes O-O bond cleavage to form singly 18-O-labeled 4. The results suggest the viability of biological Cu-1/O-2/(center dot NO) peroxynitrite formation and chemistry, that is, not coming from free superoxide plus center dot NO reaction.