THE mechanistic pathways of reactions in an electrical discharge are of interest because of their relevance, on one hand, to reactions in primordial atmospheres1–4 and, on the other, to mass spectrometric electron impact mechanisms5–8. Although simple mixtures containing methane have been studied1–4, relatively little is known about the mechanisms of electrical discharge reactions of other organic molecules9–11. We have chosen to make a detailed examination of the products and mechanisms of the reactions of isoprene in an electrical discharge, because isoprene is a relatively simple molecule which contains loosely bound π-electrons and should be capable of forming interesting higher molecular weight products, and also because of the wide occurrence of isoprenoid units in compounds of biological importance.