A theoretical study of the [C-3,HS5,N](.+) potential energy surface is presented. Ab initio molecular orbital calculations at the QCISD(T)/UMP2 level with the 6-31G(d,p) basis set show that acetonitrile N-methylide [CH3CNCH2](.+), a(.+), and N-methylketenimine [CH3NCCH2](.+), b(.+), are the most stable species among the 15 isomers considered, and a heat of formation of Delta(f)H degrees(298) = 970 kJ/mol is proposed for both species. Detailed examination of the [C-3,H-5,N](.+) potential energy surface indicates that a(.+), b(.+), and related isomers are stable and distinct species in the gas phase, isolated by energy barriers as high as 300 kJ/mol. Their neutral equivalent, a and b, have also been studied, thus allowing their adiabatic ionization energies to be estimated: IEa(a) 7.1 eV and IEa(b) 8.0 eV. Finally, the theoretical study of a(.+) and b(.+) fragmentations provides an explanation for the similarity in their high-energy CID spectra by showing a possible isomerization of these species prior to dissociation.