Radical cations of iminoethenone, RN=C=C=O-.+ (R = H and CH3), have been generated by dissociative ionization of isoxazolopyrimidine precursors and structurally characterized by collisional activation mass spectrometry. Contrasting with the behavior of their sulfur analogues, vertical electron transfer in a neutralization experiment does not result in the formation of observable RN=C=C=O neutrals. This finding is in excellent accord with high-level ab initio calculations at the G2(MP2) lever of theory. Although both HNCCO (2) and CH3NCCO (4) are calculated to be stable equilibrium structures, dissociation into HNC (or CH3NC) + CO require activation energies of only similar to 10 kJ mol(-1). Both 2 and 4 are predicted to have a singlet ground state, 78 and 83 kJ mol(-1), respectively, below the triplet. Iminoethenone radical cation (2(.+)) is predicted to be the lowest-energy C2HNO+ ion and is stable toward all possible fragmentations : the most favorable fragmentations into H-.+ NCCO+ and HNC + CO.+ are in accord with the mass spectrometric observations.