Acetonitrile N-methylide radical cations [CH3CNCH2](.+), a(.+), and N-methylketeneimine [CH3NCCH2](.+), b(.+) have been prepared by ion-molecule reactions between ionized cyclobutanone or ionized ketene and acetonitrile or methyl isocyanide. Ion structures have been identified using different experimental conditions including Fourier transform ion-cyclotron resonance experiments, high-and low-energy collisional processes, and reactions within a quadrupole (rf-only) collision cell installed in a large-scale tandem mass spectrometer. All experiments demonstate that a(.+) and b(.+) are stable distonic species that do not easily interconvert. Ab initio molecular-orbital calculations at the QCISD(T)//UMP2 level with a 6-31G(d,p) basis set show that ions a(.+) and b(.+) are of comparable stability, and a heat of formation of 970 kJ/mol is proposed for both species.