Differences between the appearance potentials of fragment ions formed a few μ secs after the ionization (so-called metastables) and normal fragment ions were measured. It is shown, however, that no immediate meaning regarding the primary fragmentation mechanism can be attached to these quantities. Instead, the experimental ionization curves of metastable and normal fragments were explained by a theoretical model which assumes 1) a linear threshold law, 2) a linear relation between the logarithm of the rate constant and the internal energy in the molecular ion, and 3) a constant density of populated states on the energy scale. In the case of HCN loss from benzonitrile ions, an excellent fit between the experimental and theoretical curves can then be achieved if it is assumed that an increase of 0.65 eV internal energy increases the rate constant by one order of magnitude. This is a surprisingly slow rise of k (E), compared with the few existing calculations in other cases.