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In this study we examined the relative importance of side reactions, i.e. the formation of volatile fatty acids (VFA), and the reduction of alternative electron acceptors (nitrate, sulfate, Fe(III)), in enhanced dechlorination microcosms, amended with different electron donors, namely lactate, butyrate, and H₂+acetate mixture. Dechlorination reactions proceeded at low rates and consequently, nearly all of the reducing equivalents (over 99%) available from electron donor consumption were channeled to (side) reactions other than dechlorination. The relevance of these side reactions was more evident with lactate which was consumed at higher rate than other electron donors. Correspondingly, high levels of VFA and soluble Fe(II) accumulated in the supernatant of lactate-amended microcosms. Ecotoxicity experiments (Lepidium sativum germination tests) also indicated that the supernatant was much more toxic/inhibitory …