Formation of negative ions under low energy electron impact (0-15 eV) is studied from C6F6and C6F5Cl at different stages of aggregation: gas phase molecules, free molecular clusters and condensed molecules. The gas phase experiments (molecules, clusters) are performed in a crossed beam apparatus and the condensed phase experiments in a UHV apparatus where the molecules are deposited in definite amounts on a cold (30 K) metallic substrate. In each case the relative cross-section for the formation of a particular anion as a function of the electron energy is recorded mass spectrometrically. Under single-collision conditions, both molecules yield parent anions from a narrow resonance at zero eV and various fragment ions from further resonances at higher energies. In clusters the low energy attachment feature broadens considerably. This is the result of effective intermolecular relaxation which strongly competes with autodetachment. Mono-meric anions are also generated from clusters in considerable amounts at higher energies (> 3 eV) via inelastic scattering from one molecule and evaporative capture of the slowed-down electron by another molecule of the same cluster (self-scavenging). In C6F5Cl clusters, dissociative attachment is strongly quenched with the exception of a new intense resonance at 6·5 eV yielding Cl−and C6F5−. At this energy, desorption of Cl−from condensed C6F6Cl also has a pronounced maximum. The 6·5 eV feature is interpreted as a core excited open channel resonance which is converted into a closed channel (Feshbach) resonance when coupled to environmental molecules, i.e. any degree of aggregation strongly quenches autodetachment in favour of dissociation.