Gas-phase enthalpies of formation (Delta H-f(g)degrees) have been determined for 40 nitrogen-containing compounds at 298 K. Three ab initio composite methods have been compared in their abilities to quantitatively determine Delta H-f(g)degrees : the G3. G3(MP2), and correlation consistent Composite Approach (ccCA) methodologies. The ccCA method resulted in a mean absolute deviation (MAD) of 1.1 kcal mol(-1) when compared to available experimental values. The comparable G3(MP2) method resulted in a MAD of 1.8 kcal mol(-1), while the G3 method resulted in a MAD of 1.2 kcal mol(-1). As a result of their comparable accuracies, the ccCA and G3 methods have been utilized to predict the Delta H-f(g)degrees) of five energetic but highly endothermic tetrazine-containing compounds with potential applications as insensitive high explosives. (c) 2010 Elsevier B.V. All rights reserved.