The ion–molecule reactions in acetylene have been studied. The reaction of C2H2+ with C2H2 yields C4H2+ and C4H3+ in the ratio 0.40:1. C2H+ ions also react to form C4H2+. Excited states of C2H2+ and possibly C2H+ are shown to be important in the ion–molecule reactions in acetylene.In acetylene–methane mixtures the following reactions have been identified and their rates measured.[Formula: see text]By studying the reactions in CH4–C2D2 mixtures the condensation reactions have been shown to involve extensive isotopic scrambling while the hydrogen-transfer reactions occur without isotopic scrambling.