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This study examined the ability of different electron donors (i.e., hydrogen, methanol, butyrate, and yeast extract) to sustain long‐term (500 days) reductive dechlorination of tetrachloroethene (PCE) in anerobic fill‐and‐draw bioreactors operated at 3:1 donor:PCE ratio (defined on a total‐oxidation basis for the donor). Initially (i.e., until approximately day 80), the H₂‐fed bioreactor showed the best ability to completely dechlorinate the dosed PCE (0.5 mmol/L) to ethene whereas, in the presence of methanol, butyric acid or no electron donor added (but low‐level yeast extract), dechlorination was limited by the fermentation of the organic substrates and in turn by H₂ availability. As the study progressed, the H₂‐fed reactor experienced a diminishing ability to dechlorinate, while more stable dechlorinating activity was maintained in the reactors that were fed organic donors. The initial diminished ability of the H₂‐fed …