Ab initio electronic structure calculations, including a natural bond orbital (NBO) analysis, are employed to compare the stabilities of larger nitrogen oxide cages and phosphorus oxide cages relative to the cage compound c-N2O3, which has been previously investigated as a potential energetic oxidizer. The larger N-O cages, c-N2O6 and c-N4O6 exhibit less internal strain but have significantly lower barriers to decomposition of 1.9 kJmol(-1) and 5.6 kJmol(-1) respectively, compared to 37.6 kJmol(-1) for c-N2O3, at the MP2/aug-cc-pVDZ level of theory. In contrast, the phosphorus oxide cage c-P2O3 exhibits similar internal strain but has a significantly larger barrier to decomposition of 40.2 kJmol(-1) compared to the 24.4 kJmol(-1) of c-N2O3 at CCSD(T)/CBS(Q-5). Furthermore, NBO analysis shows that the P-O bond is more ionic in nature compared to the N-O bond. The reduced degree of ionic character leads to the kinetic instability of the nitrogen oxide cages and therefore renders them impractical as energetic oxidizers.