The unimolecular decompositions of two isomers of [C&N]+, CH3CH2CH=NH2 and CH3CHzh=CH,, are dis-ed in terms of the potential energy profile over which reaction may be considered to occur. The energy needed to promote slow (metastable) dissociations of either ion is found to be less than that required to cause isomerhation to the other structure. This finding is supported by the observation of ditferent decomposition pathways, different metastable peak shapes for GH,, loss, the results of 'H labelling studies, and energy measurements on the ,two ions. The corresponding potential energy profile for decomposition of the oxygen analogues, CH3CHzCH=bH and CH3CH&CHZ, is compared and contrasted with that proposed for the [C,&N]' isomers. This analysis indicates that for the oxygen analogues, the energy needed to decompose either ion is very similar to that required to cause isomerhation to the other structure. Consequently, dissociation of either ion is finely balanced with rearrangement to the other and similar reactions are observed. Detailed mechanisms are proposed for loss af HzO and C& from each ion and it is shown that these mechanisms are consistent with 'H and "C labelling studies, the kinetic energy release associated with each decomposition channel, the relative competition between H,O and C2H, loss and energy measurements. Neutral lost' Metastable peak shape Ion structure C, H, NH. for C2H9 loss CH,CH,$H=kH, ( a ) 29 71 Flat-topped CH,CH,NH=CH, ( b ) 100 0 Gaussian "Abundances measured by peak areas in the first field free region (1st FFR) and normalized to a total metastable ion current of 100 units from m / e