Collisional activation mass spectrometry (CA) reveals the existence of 11 stable C 5 H 9 + cations in the gas phase, e. g. the substituted allyl cations a, b, c, d,and e, the sub-stituted vinyl cations f, g, h, and i, the methyl cyclobutyl cation j and the cyclopentyl cation k, respectively. The ethyl substituted allyl cation a is formed via dissociative ionization of the isomeric precursors 1, 3, 4, 5, 18, 19, 20, 22, and 28 by means of various mechanistic processes, whereas the 1,3-dimethylallyl cation b is generated from both 2 (by allylic cleavage) and in part from the stereoisomeric cyclopropan derivatives 25, 26 and 27. 6 + -gives a mixture of the vinyl cations 1 and g. From 13 and 14 the main product generated is the 1,2-dimethylallyl cation d, which is formed directly from 11 and also by quite complicated processes from 13, 14 and to a certain extent from 25, 26 and 27. The dissociative ionization of 9, 15, 16, 21, 24 and (in part) 23 give rise to the formation of the substituted vinyl cation h. Decomposition of 23 + • results not only in formation of h but generates also the 1,1-dimethylallyl cation e. From 29 + -and 30 + -both the methylcyclo-butyl cation j and cyclopentyl cation k are produced, whereas the isomeric precursor 28 gives mainly the substituted allyl cation a and a second, as yet, unidentified C 5 H 9 cation. In general, it can be stated that the gas phase chemistry of cation radicals of substituted cyclopropanes is characterised by multistep-reactions, commencing with spontaneous ring opening. The so formed intermediates undergo various rearrangements (including hydrogen and alkyl shifts) prior to expulsion of Br·. Direct elimination of Br· from intact cyclopropan-like structures, followed by ring opening of the intermediate cyclopropyl cation, cannot compete with the above-mentioned multistep-sequences.