Abstract By combining results from a variety of mass spectrometric techniques (metastable ion, collisional activation, collision‐induced dissociative ionization, neutralization‐reionization spectrometry, 2 H, 13 C and 18 O isotopic labelling and appearance energy measurements) and high‐level ab initio molecular orbital calculations, the potential energy surface of the [CH 5 NO] + ˙ system has been explored. The calculations show that at least nine stable isomers exist. These include the conventional species [CH 3 ONH 2 ] + ˙ and [HOCH 2 NH 2 ] + ˙ , the distonic ions [OCH 2 NH 3 ] + ˙ , [ONH 2 CH 3 ] + ˙ , [CH 2 O(H)NH 2 ] + ˙ , [HONH 2 CH 2 ] + ˙ , and the ion‐dipole complex CH 2 NH 2 + …︁ OH˙. Surprisingly the distonic ion [CH 2 ONH 3 ] + ˙ was found not to be a stable species but to dissociate spontaneously to CH 2 O + NH 3 + ˙ . The most stable isomer is the hydrogen‐bridged radical cation [HCO …︁ H …︁ NH 3 ] + ˙ which is best viewed as an immonium cation interacting with the formyl dipole. The related species [CH 2 O …︁ H …︁ NH 2 ] + ˙ , in which an ammonium radical cation interacts with the formaldehyde dipole is also a very stable ion. It is generated by loss of CO from ionized methyl carbamate, H 2 NC(O)OCH 3 and the proposed mechanism involves a 1,4‐H shift followed by intramolecular ‘dictation’ and CO extrusion. The [CH 2 O …︁ H …︁ NH 2 ] + ˙ product ions fragment exothermically, but via a barrier, to NH 4 + ˙ HCO …︁ and to H 3 NC(H)O + ˙ H˙. Metastable ions [CH 3 ONH 2 ] +…︁ dissociate, via a large barrier, to CH 2 O + NH 3 + + and to [CH 2 NH 2 ] + + OH˙ but not to CH 2 O + ˙ + NH 3 . The former reaction proceeds via a 1,3‐H shift after which dissociation takes place immediately. Loss of OH˙ proceeds formally via a 1,2‐CH 3 shift to produce excited [ONH 2 CH 3 ] + ˙ , which rearranges to excited [HONH 2 CH 2 ] + ˙ via a 1,3‐H shift after which dissociation follows.