The reaction of NH with NO2, which can produce N2O + OH and HNO + NO by two distinct reaction paths, has been studied by ab initio molecular orbital calculations. The first reaction path taking place by initial N-N association forms an intermediate HNNO2, 1, which undergoes H-migration yielding NN(O)OH, 3, before reaching the N2O + OH product. The transition state 2 for the rate-determining 1 --> 3 rearrangement, with the activation barrier of 30 kcal/mol at the G2-level of calculation, lies below the energy of the reactants. The O migration for the HNNO2 1 intermediate to produce HNO + NO is inaccessible at low temperatures due to the presence of a high migration barrier. The second path via initial N-O association forms an intermediate HNONO, 9, which is expected to dissociate readily to HNO + NO via a loose transition state lying 24 kcal/mol below the reactants. Since the initial N-N and N-O association reactions effectively occur with no barriers, the overall activation energy for NH + NO2 is expected to be negligible or slightly negative as was found experimentally.