Abstract The acetylene discharge is characterized by the formation of a visible precipitate indicating a high rate of conversion to non-volatile products. The principal ionic condensation reaction can be summarized by the reaction sequence: C 2H x + + C 2H 2 → C 4H y + + C 2H 2 → C 6H z + + C 2H 2, etc. Axially sampled ions are characteristic of more energetic ionization processes than radially sampled ions; the latter result more from ion-molecule condensation reactions than the former. Non-ionic gaseous products found were hydrogen and diacetylene. The ion chemistry closely parallels that observed by high pressure mass spectrometry. The degree of stabilization and subsequent break-down of chemical intermediates formed from an ion and an acetylene molecule largely determine the ion chemistry in the discharge. The greater the state of excitation of the reacting ion, the more unsaturated the product ion is likely to be. Discharges of mixtures of acetylene with helium, argon or xenon, respectively, show no evidence of the rare gas altering the chemistry by the exchange of potential energy from either long-lived metastable states or charge exchange. Only in the case of the axially sampled xenon-acetylene mixture is there any evidence of an “excess energy” reaction occurring with acetylene ions. Isotropic scattering from xenon allows a gain of about 6 eV in random kinetic energy from the oscillating electric field in the RF sheath. In argon the gain is only 3.5 eV. In helium the random energy gained from the RF field is much smaller and the trajectory of ions across the sheaths must be nearly linear. The ions sampled in either direction from a helium mixture are more representative of plasma products than sheath products. Thus helium behaves as a true diluent in that it does not participate measurably in the ion chemistry either electronically or kinetically.