Organic ions decompose in the field-free regions of a mass spectrometer after a relatively well-defined lifetime (t ≈ 10−5s). This lifetime is very long compared with the time-scale of molecular bond vibrations and consequently the excess internal energy (“non fixed” energy) present in the transition state for dissociation is comparable with that found in solution experiments. In general, only the energetically most favorable decay routes are able to compete. Moreover, information concerning kinetic energy release (T) accompanying unimolecular decomposition may be obtained from a consideration of the metastable peak for the process; these extremely valuable data are inaccessible to solution experiments because of the effects of collisions. The chemistry of ions may therefore be investigated conveniently by generating the ions of interest in a mass spectrometer and analyzing the reactions which occur in metastable transitions.