Low-energy (0–15 eV) electron-stimulated desorption of O − from multilayer O 2 proceeds via different O 2 −* resonances at an onset energy near 6 eV. In contrast, O 2 deposited on an acetonitrile (CD 3CN) substrate shows a significant contribution towards lower energy with an onset near 2.6 eV. The absolute intensity of this low-energy signal has a maximum at O 2 coverages below 1 monolayer, suggesting a substrate-mediated reaction. It is proposed that the ( 2 Π CN ) resonance of acetonitrile acts as a low-energy electron acceptor. The energy required for low-energy desorption of O − is then provided by a reaction of the excited anionic complex (CD 3CN·O 2) −* yielding O − and a thermodynamically favourable neutral species like methylisocyanate (CD 3NCO).