The normalized unimolecular rate constant for loss of a methyl radical from pent-3-en-2-01 molecular ions with lifetimes ranging from lo-" to loM9 s was studied by field ionization kinetics (FIK). The normalized rate curve shows maxima at molecular ion lifetimes of and lo-".' s. Based on results for deuterium and "C-labelled pent-3-en-2-01, the maximum at s is ascribed to loss of the methyl group at the 1-position by a direct cleavage reaction. The maximum at lo-'*.' s is attributed to a 1,ZH shift-initiated rearrangement of the molecular ion, which leads to loss of the methyl group at the 5. and 1-positions. The time dependence of the processes in the form of the maxima on the normalized rate curve is discussed qualitatively in terms of a lower critical energy and a tighter transition state of the 1,2-H shift than those of the diect cleavage reaction. Collision-induced dissociation of the [ C,H,O] + product ions in combination with FXK provides evidence that at molecular ion lifetimes corresponding to the first maximum on the rate curve protonated crotonaldehyde is formed, whereas protonated methyl vinyl ketone and the butyryl cation are formed at times corresponding to the second maximum.