Abstract High level ab initio calculations at the G2(ZPE = MP2) level have been used to characterize the potential energy surfaces for rearrangement/fragmentation of various [C 3H 8N] + and [C 3H 7S] + isomers. In contrast to the behavior in the corresponding [C 3H 7O] + system, it is found that ion–neutral complexes are only of minor importance in determining the fragmentation characteristics. Either dissociation of such complexes occurs too fast due to a large barrier to their formation ([C 3H 8N] + system), or alternative lower-energy rearrangement routes that do not involve ion–neutral complexes are available ([C 3H 7S] + system). Calculated thermochemical quantities such as heats of formation and reaction barriers are found to be in reasonable agreement with experimental results. Metastable ion product abundances and results of both deuterium- and 13C-labeling experiments are rationalized in terms of the calculated potential energy surfaces and rate constants obtained using Rice-Ramsperger-Kassel-Marcus theory.