DSBN+, a conjugated oligoelectrolyte (COE), was added to microbial electrolysis cells (MECs) to improve hydrogen recovery. The volume of hydrogen gas recovered in a fed-batch cycle of mixed culture MECs increased by 126x compared to controls (no COE addition), mainly by preventing the loss of hydrogen to methane production. Performance in pure culture MECs fed with Geobacter sulfurreducens increased by factors of 10.5 in terms of energy yield, 2.1 in COD removal, and 11.8 in hydrogen yield. Hydrogen gas recycling was reduced, and the volume of hydrogen gas recovered increased by 6.5x compared to controls. Minimal methane production and a lack of hydrogen gas uptake by G. sulfurreducens suggested that the COEs increased hydrogen recoveries by interfering with hydrogen uptake by hydrogenotrophic methanogens but also by exoelectrogenic bacteria. COEs may therefore be useful for inhibiting the activities of certain hydrogenases, although the mechanism of inhibition needs further investigation. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.