Metastable peaks for the fragmentation C2H5N+• → C2H4N+ + H• have been studied in the first field-free region of a double focussing mass spectrometer. Isomeric structures of C2H5N+• ions derived from a variety of precursor molecules were characterised by their metastable peak shape and kinetic energy release. The identifications were aided by appropriate deuterium labelling experiments. Four isomeric ionsr [Formula: see text]were shown to retain their structural identity within the ion energy and lifetime ranges observable in these experiments. Ion (a), generated from aziridine was found to be indistinguishable from its open chain isomer [Formula: see text]The results closely parallel those previously found for the isoelectronic C2H5N+• ions, but the differences in characteristic kinetic energy release are smaller. A simple deuterium exchange labelling experiment may be necessary for the unequivocal identification of C2H5N+• ion structures.