Kinetics and mechanism for the bimolecular decomposition of HONO have been studied by ab initio molecular orbital (G2M) and transition-state theory calculations. The reaction can take place by the interaction of a cis and a trans isomer or two cis or two trans isomers, via four-, five-, and six-member ring transition states, with decreasing reaction barriers as the size of the ring increases. The lowest energy path with a 13.7 kcal/ mol barrier was found to occur by the six-member ring TS1 formed by the reaction of cis- and trans-HONO. A similar six-member ring TS (TS2) formed by two cis isomers has a barrier height of 15.1 kcal/mol, which is very close to the 5-ring TS formed by two trans isomers, 15.7 kcal/mol. The total rate constant computed with the ab initio MO results, including the three reaction channels mentioned above and an additional channel involving a five-member ring TS formed by a cis- and a trans isomer with a 17.7 kcal/mol barrier, can be represented by the three-parameter expression for the 300-5000 K temperature range: k = 5.8 × 10-25T3.64 exp(-6109/T) cm3/(molecule's), which includes the Boltzmann-averaged contribution of the cis isomer. The theoretical value was found to be considerably lower than the available experimental results (which are believed to have suffered from deleterious surface effects).