Fulminates containing the CNO- ion have been widely utilized as high-energy density materials (HEDMs) for more than 120 years. Yet no purely covalently bound CNO molecule, i.e., nitrile oxide, is known to behave as an HEDM. In this study, we performed a thorough investigation of the potential energy surface of nitrile oxide ONCNO and related isomers, applying various sophisticated methods including G4, CBS-QB3, W1BD, CCSD(T)/CBS, and CASPT2/CBS. The Gibbs free energy calculations showed that the decomposition of ONCNO to the considerably endothermic products CNO + NO is favored compared to that into the highly exothermic products CO2 + N-2. Thus, ONCNO fails to be the long expected nitrile oxide HEDM. However, with the rate-determining barrier of 23.3 kcal mol(-1) at the W1BD level, ONCNO should be experimentally accessible.