The unimolecular decomposition of HNNO2 and the related bimolecular reaction of NH with NO2 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 HNNO2 and HNONO, the formation of exothermic products N2O and OH via the HNNO2 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 N2O 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%.