The interaction of low energy electrons with stable and unstable molecules has been investigated with particular emphasis on negative ions formed in dissociative electron attachment. The experiment includes a trochoidal electron monochromator and a time-of-flight mass spectrometer. Measurements of dissociative electron attachment to HCCCN have been made inspired by the presence of this molecule in many extraterrestrial environments and by recent theoretical calculations of electron attachment to HCCCN. Two regions of electron attachment were observed with formation of CCCN- at 1.56 eV and CCCN-, CN-, HCC- and CC at - 5.30 eV where 2 II shape resonances were predicted to be located. Quantum chemical calculations performed in this work confirm these dissociation processes follow electron attachment into empty rr* orbitals. Negative ions were also observed at higher energies due to core excited resonances. Metastable dissociation of SF6-* has been observed in competition with autodetachment after attachment of close to 0 eV electrons to SF6. The lifetime of SF6-* has been estimated from the present measurements as a function of electron energy and hence, the internal energy of the anion. Metastable dissociation of a number of substituted benzene rings, such as dinitrobenzene, has also been investigated. This appears to be the first experimental study of the metastable decay of molecular anions with known internal energies. Measurements of dissociative electron attachment to the unstable CS and molecules have been performed to obtain total cross sections for dissociative electron attachment and characteristic negative ion mass spectra 'fingerprints'. CS forms S- at 5.43 eV, Cat - 6.40 eV and S- at - 6.70 eV. Dissociative electron attachment to CF2 was not observed, and an upper limit of the absolute cross section for the formation of F+CF has been deduced. A potential application of dissociative electron attachment measurements to plasma diagnostics is considered.