It was recently suggested that HOONO, which forms after protonation of the ONOO- anion under biological conditions, decomposes into HNO and ((1)Delta(g))O-2. Subsequent workers argued that the mechanism for HOONO decomposition proceeds via homolytic bond fission, producing the radical pair OH and NO2, and another recent study argued that a cyclic form of the peroxynitrous acid results in the products H+ + O-2((1)Delta(g)) + NO-. Calculations on the reaction pathway for the process showed that it required a high activation energy, and is thus implausible. High level ab initio molecular orbital theory including extrapolation to the complete basis set limit has been used to calculate the heats of formation of reactants and products for the homolytic bond fission pathways for decomposition of HOONO and the molecular pathway that yields HNO and O-1(2) as well as the transition state for the latter process. These data are used to evaluate the probability of whether the decomposition of peroxynitrous acid can produce HNO and O-1(2). Isomerization of HOONO to HONO2 is also discussed.