Abstract The kinetic behaviour of rearrangement reactions and direct bond cleavages as derived qualitatively from FI mass spectra is summarized. At low excitation energies (at 12 eV EI, or under FI conditions) skeletal rearrangements are slow, and hydrogen migration relatively fast, decomposition processes (decomposition times corresponding to the peak maxima: several times 10 −6 sec, and several times 10 −11 sec, respectively). Among direct bond fissions alkyl radical eliminations are especially slow processes. An unambiguous differentiation between rearrangements and direct bond cleavages from the kinetic data is difficult. The kinetic study of the tropylium ion formation demonstrates that this rearrangement starts with a field-induced benzyl ion formation in an FI source (in contrast to EI conditions).