The structures, energetics, and decomposition mechanisms of gaseous ammonium nitrite (NH4NO2) and ammonium dinitramide [ADN, NH4N(NO2)2] have been studied theoretically by different ab initio molecular orbital approaches. In the gas phase, both species have the structures of molecular complexes, [NH3]·[HX]. The ionic geometries, [NH4 +][X-], are not local minima on the potential energy surface and would not be stable after vaporization. For NH4NO2, [NH3]·trans-HONO] is the most stable isomer, and [NH3]·[cis-HONO] and [NH3]·[NHO2] structures lie higher by 1.4 and 8.4 kcal/mol at the G1 level of theory. For the gaseous ADN, [NH3]·[HN(NO2)2 is the most stable structure, while the [NH3]·[HON(O)NNO2] isomer is 2.3 kcal/mol less favorable. The calculated dissociation energies of the [NH3]·[HX] complex to NH3 and HX are 8-9 and 12-14 kcal/mol for NH4NO2 and ADN, respectively. The energies for elimination of the NO2 group from HN(NO2)2 and HON(O)NNO2 are found to be 38-40 kcal/mol, while the barrier for HON(O)NNO2 dissociation is about 42 kcal/mol. We predict the following values of the heats of formation, ΔHf°(0), in the gas phase: -35.5 kcal/mol for [NH3]·[trans-HONO] and 3.2 kcal/mol for [NH3]·[HN(NO2)2]. A realistic mechanism for the decomposition of ADN, which is fully consistent with the products measured by Brill et al. (ref 3), has been proposed on the basis of these ab initio MO results.