Ammonia oxide (NH(3)O), a zwitterionic tautomer of hydroxylamine (NH(2)OH), has been identified to explain the high reactivity of NH(2)OH. Recent evidence has shown that ammonia oxide exists in the condense phase and aqueous solution. However, neither experimental nor theoretical data of its thermochemical properties are presently available. In this work, thermochemical parameters of ammonia oxide are calculated, under standard conditions, using isodesmic reactions at several theoretical methods (HF, MP2, B3LYP, G2, G2MP2, G3, G3B3, and CBS-Q) and several basis sets (Dunning correlation-consistent and Pop le-style). To monitor the computed values, the values of hydrogen peroxide are calculated by the same methods and compared with the experimental data. The quantum chemistry calculations predict the value of molar enthalpy of formation to be (55.7 +/- 2.9) kJ . mol(-1) and molar Gibbs energy of formation to be (103.8 +/- 2.9) kJ . mol(-1), for gaseous ammonia oxide at 1 atm and 298.15 K. The determined molar entropy and molar heat capacity at constant volume, also at 1 atm and 298.15 K, are (221.1 +/- 0.1) J.mol(-1).K(-1) and (28.9 +/- 0.3) J.mol(-1).K(-1), respectively. We have demonstrated that molecular simulation is a powerful tool to obtain thermodynamic properties of unstable compounds, such as ammonia oxide.