The title ions have been generated by dissociative ionisation of appropriate precursor molecules and have been unequivocally identified by collisional activation and collisional ionisation mass spectrometry. Their heats of formation, [Formula: see text], have been measured and were found to be similar to those for their ionic isomer of conventional structure, [CH3X]+•. The exception is [CH2NH3]+• whose [Formula: see text], is ca. 30 kcal mol−1 higher than that of [CH3NH2]+•. The shapes and appearance energies of metastable peaks in conjunction with isotopic labelling experiments, show that these novel ions do not interconvert with their conventional isomer at internal energies below the threshold for their fragmentation of lowest energy requirement, namely, loss of H•. However, unimolecular H• loss from all [CH2XH]+• ions is preceded by a rate determining isomerisation to [CH3X]+•, followed by fast dissociation. The results are generally in good agreement with recent abinitio calculations. Also described are the properties of the homologous ion [CH2CH2NH3]+•.