A study with H2-based membrane biofilm reactors (MBfRs) was undertaken to examine the effectiveness of direct H2 delivery in ex-situ reductive dechlorination of chlorinated ethenes. Trichloroethene (TCE) could be reductively dechlorinated to ethene with up to 95% efficiency as long as the pH-increase effects of methanogens and homoacetogens were managed and dechlorinators were selected for during start-up by creating H2 limitation. Based on quantitative PCR, the dominant bacterial groups in the biofilm at the end of reactor operation were Dehalococcoides, Geobacter, and homoacetogens. Pyrosequencing confirmed the dominance of the dechlorinators and identified Acetobacterium as the key homoacetogen. Homoacetogens outcompeted methanogens for bicarbonate, based on the effluent concentration of acetate, by suppressing methanogens during batch start-up. This was corroborated by the methanogenesis functional gene mcrA, which was 12 orders of magnitude lower than the FTHFS functional gene for homoacetogens. Imaging of the MBfR fibers using scanning electron microscopy showed a distinct Dehalococcoides-like morphology in the fiber biofilm. These results support that direct addition of H2 can allow for efficient and complete reductive dechlorination, and they shed light into how H2-fed biofilms, when operated to manage methanogenic and homoacetogenic activity, can be used for ex-situ bioremediation of chlorinated ethenes. Biotechnol. Bioeng. 2012;109: 22002210. (c) 2012 Wiley Periodicals, Inc.