Peroxynitric acid/peroxynitrate (PNA) rivals peroxynitrous acid/peroxynitrite (PNI) in importance as a reactive oxygen species. These species possess similar two-electron oxidative behavior. On the other hand, stark differences exist in the stability of these molecules as a function of pH and in the presence of CO2, and also in the types of bond homolysis reactions that PNA and PNI may undergo. Using CBS-QB3 theory, we examine these similarities and differences. The activation barriers for two-electron oxidation of NH3, H2S, and H2C=CH2 by PNA and PNI are found to be generally similar. The O-O BDE of O2NOOCO2- is predicted to be 26 kcal/mol greater than that of ONOOCO2-. This accounts for the insensitivity of PNA to the presence of CO2. Likewise, the O-O BDE of O2NOOH is predicted to be 19 kcal/mol greater than that of ONOOH, in excellent agreement with experiment. The fundamental principle underlying the large differences in O-O BDEs between PNA and PNI species is that the NO2 that is formed from PNI can relax from the B-2(2) excited state to the (2)A(1) ground state, whereas no such comparable state change occurs with NO3 from PNA. Decomposition of the anions OxNOO- is more complex, with the energetics influenced by solvation. ONOO- can homolyze to yield NO/O-2(-); however, this pathway represents a thermodynamic "dead end" since the radical pairs generated by homolysis should mostly revert to starting material. However, decomposition of O2NOO-yields the stable products NO2-/O-3(2), a couple that is more stable than starting material. This may occur either by initial formation of NO2/O-2(-) or NO2-/O-1(2), with the latter intermediates thermodynamically favored both in the gas phase and in solution. Given the extremely fast back-reaction of the homolysis products, heterolysis probably dominates the observed O2NOO- decomposition kinetics. This is in agreement with the first of two "kinetically indistinguishable" mechanistic possibilities proposed for O2NOO- decomposition (Goldstein, S.; Czapski, G.; Lind, J.; Merenyi, G. Inorg. Chem. 1998, 37, 3943-3947).