From the monotonic increase of the average initial energy of paraffin fragment ions formed by electron impact the existence of unimolecular dissociation sequences is concluded as it is assumed by the statistical theory of mass spectra. For primary decomposition steps the total kinetic energies set free during the respective dissociation processes (transition energies) have been deduced from measured initial energies. The transition energies obtained in this way may be compared with theoretical data for the translational energies in the transition states (statistical energies) as calculated by means of the statistical theory of mass spectra. In case of—C bond cleavage theoretical data are close to the experimental values (≈ 0.1 ev). In case of a C—H bond cleavage, however, theoretical values are always lower than the experimental ones. In rearrangement reactions an activation energy for the reverse process may exist, which should show up, at least partly, in the transition energy. For a primary H2-abstraction process the activation energy for the reverse process aE can be calculated from thermochemical data. The observed transition energies are always lower than the calculated values. A C—C skeleton rearrangement also shows some influence of aΕ on the transition energy. The absolute effect, however, is small—less than 0.1 ev. For hydrogen shift reactions no indication of an aE-contribution was found.