The ion–molecule reactions in ethylene have been studied under conditions where C2H4+ was the only primary ion. In addition to the expected secondary ions C3H5+ and C4H7+, the C3H4+, C2H2+, and C2H3+ ions have been detected, presumably formed by the reactions[Formula: see text]Reactions [b] and [c] are endothermic for ground state reactants and it has been shown that the kinetic energy of the reactant ion plays a major role in determining the relative importance of these endothermic reactions.The C5H9+ ion has been shown to originate by further reaction of the C3H5+ secondary ion rather than by reaction of the (C4H8+)* collision complex. The reversion of the (C4H8+)* complex to reactants has been confirmed by the observation of isotopically mixed ethylene ions in C2H4–C2D4 mixtures.