Ab initio molecular orbital theory is used to determine the molecular structure of ammonia oxide (NH3O). It is found that the N-O bond length is considerably overestimated by minimal (STO-3G), split-valence (4-31G and 6-31G) and large sp basis sets. This fault is rectified when polarization functions on nitrogen and oxygen are included in the basis (6-31G*) leading to the best theoretical value for the N-O length of 1.377 A. Electron correlation has little effect on the calculated bond length. Calculations (STO-3G and 4-31G) are also reported for trimethylamine oxide.