In the present study, we investigated the use of a lab-scale, bio-electrochemical system to generate H-2 from protons reduction at controlled cathodic potentials, in support of the microbial reductive clechlorination of trichloroethene (TCE). Several batch potentiostatic experiments were performed on graphite cathodes at different potentials ranging from -0.700 to -1.000Vvs. Ag/AgCL By appropriately changing the applied cathode potential it was possible to finely control the liquid phase H-2 concentration, which resulted from a balance between H-2 generation (from protons reduction) and consumption (from clechlorination and methanogenesis). Microbial TCE clechlorination was stimulated when the potential applied to the graphite cathode was lower than -0.800 V vs. Ag/AgCL However, a combination of high dechlorination rate and high current efficiency was achieved only in a very narrow range of cathode potentials (i.e., -0.850 to -0.875 V vs. Ag/AgCl). Methane formation was significantly slower than TCE clechlorination, probably due to the presence in the mixed culture of a lower number of methanogens compared to dechlorinators. In spite of this fact, these two competing metabolisms were stimulated in a similar way by the application of an external potential, thereby indicating a similar affinity for H2. Indeed, calculated half-velocity coefficients for H-2 for clechlorination and methanogenesis were 20.1 +/- 7.6 and 17.9 +/-8.5 nM, respectively. (c) 2008 Elsevier Ltd. All rights reserved.