The γ-radiolysis of acetonitrile has been studied at room temperature over a wide range of radiation dose and dose rate, and also in the presence of scavengers for radicals and for thermal electrons. The main product is a short-chain polymer (G = 4.2 molecules of CH 3 CN per 100 eV of absorbed radiation). Other important products are hydrogen, methane, succinonitrile, and hydrogen cyanide, with smaller amounts of other hydrocarbons and nitriles. From the observed variation of product yields with dose and with scavenger concentration a mechanism is proposed in which the products of the primary ionization, <tex-math>${\rm CH}_{3}{\rm CN}^{+}$</tex-math> and e - , are precursors of the radicals <tex-math>${\rm CH}_{2}{\rm CN}$</tex-math> and CH 3 , respectively, while recombination of ionic species produces "hot" H atoms. Observations of electron spin resonance spectra in the γ-irradiated solid at low temperatures are cited to support the proposed mechanism.