The unimolecular decomposition of HNNO 2 and the related bimolecular reaction of NH with NO 2 have been studied by high-level ab initio molecular orbital and statistical theory calculations. The potential energy surface for the bimolecular association leading to the formation of HNNO 2 and HNONO, the formation of exothermic products N 2 O and OH via the HNNO 2 intermediate, and of HNO and NO via the HNONO intermediate have been computed with a modified GAUSSIAN 2 (G2M) method. The rate constants for these two bimolecular reaction channels have been calculated by means of the canonical variational Rice-Ramsperger-Kassel-Marcus (RRKM) approach over a broad temperature range. The predicted values at room temperature correlate reasonably well with the experimental overall rate and also with the experimental product branching probability for the formation of N 2 O vs HNO via the nitro and nitrite intermediates. The effect of multiple reflections within the potential well of the HN-ONO complex was found to be quite significant. It reduces the rate constant for the formation of the HNO+NO products by as much as 350%.