AbstractReactions in negatively charged clusters following resonant electron attachment have been studied in a supersonic beam experiment. The results are discussed with respect to the correspondent processes from single molecules in the gas phase and from condensed molecules. It is shown that electron capture by oxygen clusters allows the population of electronic states in O2− not accessible in the single‐electron‐molecule frame of reference (violation of the σ− selection rule). Clusters composed of O2/CF2Cl2 yield ionic products composed of both constituents. They are generated along a complex scattering process within the ionized aggregate. The resonance profiles of their ion yields contain information on their genesis, i.e. the initial step of electron capture by the electrophore in the cluster. Since these reactions occur predominantly at low energies, sometimes at zero eV, we consider them as highly relevant in any environment (gaseous or condensed) where low energy electrons are present.