The rate constant curves k(E) for the formation Of C7H7+ ions by methyl loss from energy selected ethylbenzene and o-xylene ions have been determined over a range from 10(3) s(-1) to 5 x 10(6) s(-1) in a reflecting ion time-of-flight mass spectrometer. The observed threshold for methyl loss from ethylbenzene ions is 3 kJ/mol lower than that in o-xylene ions. The slope of the rate constant curves is very different at the threshold, indicating that isomerization of the precursors to a common intermediate does not play an important role. From the analysis we conclude that the C7H7+ ions formed at the threshold have the structure of benzylium ions for ethylbenzene but that of tropylium ions for the o-xylene. At higher energies both C7H7+ isomers appear to be formed from ethylbenzene ions.