Mass-spectrometric experiments were combined with ab initio calculations to explore the cationic and neutral [H-3,N,O](.+/0) potential energy surfaces and relevant anionic species. The calculations predict the existence of three stable cationic and neutral [H-3,N,O](.+/0) isomers, i.e, ammonia oxide H3NO.+/0 (1(.+/0)), hydroxylamine HNOH2.+/0 (3(.+/0)) and the imine-water complex HNOH2.+/0 (3(.+/0)). Hydroxylamine 2 represents the most stable isomer on the neutral surface (E-rel = 0), and the metastable isomers 1 (E-rel = 24.8 kcal mol(-1)) and 3 (E-rel = 61.4 kcal mol(-1)) are separated by barriers of 49.5 kcal mol(-1) and 64.2 kcal mol(-1), respectively. Adiabatic ionization of 2 (IEa = 9.15 eV) yields 2(.+), which is 21.4 kcal mol(-1) more stable than 1(.+) and 36.4 kcal mol(-1) more stable than 3(.+). The barriers associated with the isomerizations of the cations are 58.6 kcal mol(-1) for 2(.+) --> 1(.+) and 71.4 kcal mol(-1) for 2(.+) --> 3(.+). Collisional activation (CA) and unimolecular decomposition (MI) experiments allow for a clear distinction of 1(.+) from 2(.+). Besides, neutralization/reionization (NR) experiments strongly support the gasphase existence of the long-sought neutral ammonia oxide.