The reactions of ten metastable immonium ions of general structure R'R2C=NH+C4H9 (R' = H, R2 = CH, , C2H, ; R' = R2 = CH,) are reported and discussed. Elimination of C4H, is usually the dominant fragmentation pathway. This process gives rise to a Gaussian metastable peak; it is interpreted in terms of a mechanism involving ion-neutral complexes containing incipient butyl cations. Metastable immonium ions containing an isobutyl group are unique in undergoing a minor amount of imine (R'R'C=NH) loss. This decomposition route, which also produces a Gaussian metastable peak, decreases in importance as the basicity of the imine increases. The correlation between imine loss and the presence of an isobutyl group is rationalized by the rearrangement of the appropriate ion-neutral complexes in which there are isobutyl cations to the isomeric complexes containing the thermodynamically more stable tert-butyl cations. A sizeable amount of a third reaction, expulsion of C,H,, is observed for metastable n-C4H9+NH=CR1RZ ions; in contrast to C,H, and R'R2C=NH loss, C,H, elimination occurs with a large kinetic energy release (40-48 k J mol-') and is evidenced by a dish-topped metastable peak. This process is explain& using a two-step mechanism involving a 1,lhydride shift, followed by cleavage of the resultant secondary openchain cations, CH,CH+CH2CH,NHCHR'R2.