ONOONO has been proposed as an intermediate in the oxidation of nitric oxide by dioxygen to yield nitrogen dioxide. The O-O bond breaking reactions of this unusual peroxide, and subsequent rearrangements, were evaluated using CBS-QB3 and B3LYP/6-311 G* hybrid density functional theory. The three stable conformers (cis,cis-, cis,trans-, and trans, trans-ONOONO, based on the O-N-O-O dihedral angles of either -0degrees or similar to180degrees) are predicted to have very different O-O cleavage barriers: 2.4, 13.0, and 29.8 kcal/mol, respectively. These large differences arise because bond breaking leads to correlation of the nascent NO2 fragments with either the ground (2)A(1) state or the excited B-2(2) state of NO2, depending on the starting ONOONO conformation. A cis-oriented NO2 fragment correlates with the (2)A(1) state, whereas a trans-oriented NO2 fragment correlates with the B-2(2) state. Each NO2 fragment that correlates with (2)A(1) lowers the O-O homolysis energy by similar to15 kcal/mol, similar to the similar to17-25 kcal/mol (2)A(1) --> B-2(2) energy difference in NO2. Hence, this provides an unusual example of conformation-dependent electronic state selectivity. The O-O bond homolysis of cis,cis-ONOONO is particularly interesting because it has a very low barrier and arises from the most stable ONOONO conformer, and also due to obvious similarities to the well-known {3,3}-sigmatropic shift of 1,5-hexadiene, i.e., the Cope rearrangement. As an additional proof of our state selectivity postulate, a comparison is also made to breakage of the O-O bond of cis,cis-formyl peroxide, where no significant stabilization of the transition state is available because the (2)A(1) and 2132 states of formyloxy radical are near-degenerate in energy. In the case of trans, trans-ONOONO, the O-O bond breaking transition state is a concerted rearrangement yielding O2NNO2, whereas for cis,cis-and cis,trans-ONOONO, the initially formed NO2 radical pairs can undergo further rearrangement to yield ONONO2. It is proposed that previous spectroscopic observations of certain N=O stretching frequencies in argon-matrix-isolated products from the reaction of NO with O-2 (or O-18(2)) are likely from ONONO2, not the OONO radical as reported.