High-level molecular orbital calculations have been performed in the framework of the G2M method to explore the reactivity between NH3 and HNO3, key molecular reactants in ammonium nitrate and ammonium nitramide systems. Two nonionic molecular reaction channels have been identified with a similar reaction barrier, 46 kcal/mol. One channel occurring via a four-member-ring transition state produces H2NNO2 + H2O (1), and the other, taking place via a five-member-ring transition state, yields H2NONO + H2O(2). A transition-state theory calculation employing the predicted energies and molecular parameters gave rise to the rate constants k(1) = 0.81T(3.47)e(-21670)/T and k(2) = 23.2T(3.50)e(-22610)/T for the temperature range 300-3000 K in units of cm(3)/(mol.s). In addition to the reactants, products, and transition states associated with the two reaction channels, several local minima (or molecular complexes) and secondary reaction products derived from the structural rearrangement of some of the molecular complexes, such as H3NO and H2NOH, have been identified and their energies calculated at the G2M level of theory.