In a previous ¹H-NMR kinetic study we examined the thermolyses of N-4-substituted-benzyl-N-nitrosopivalamides (substituents = MeO, Me, H, and NO₂) and N-benzyl-N-nitrosotosylamide in a range of solvents and at various temperatures. Activation parameters for these decompositions varied systematically with the nature of the 4-substituent, solvent polarity, and the acid moiety on the nitrosoamide. These observations were interpreted in terms of the presence of a high-energy, charge-separated oxadiazetyl moiety occurring early in nitrosoamide thermolyses. The age (early transition state or late intermediate) of this entity was unknown and consequently the identity of the true first step of nitrosoamide thermolyses remained undefined. In the current work, density functional calculations of these first order decompositions were conducted; these studies confirm the presence of the oxadiazetyl species along the reaction pathway and identify them as transition states between the N-nitrosoamide and the trans-diazoester establishing conclusively that the latter species is the first intermediate along the reaction pathway. Except for minor discrepancies, the calculated activation parameters agree well with the experimentally-derived data.