Reductive dehalogenation of aromatic compounds at a mercury cathode in dimethylformamide containing 1% deuterium oxide with tetra-n-propylammonium perchlorate as supporting electrolyte may or may not result in deuterium incorporation, depending on the substrate. Compounds giving a radical anion intermediate which is detectable by cyclic voltammetry do not incorporate deuterium. The radical anion decomposes away from the electrode to a σ-radical which abstracts a hydrogen atom from the organic solvent. Compounds which give no detectable radical anion accept one electron in the initial step and fragment to a σ-radical at the electrode surface. This radical is reduced further to a carbanion which reacts with incorporation of deuterium into the product. Evidence for σ-radical intermediates in the latter cases comes from isolation of dimers or products of intramolecular radical cyclisation of appropriate substrates.