With limited external applied voltage, the microbial electrolysis cell (MEC) could produce hydrogen by exoelectrogenic microorganisms. The present study revealed that a cubiodshaped chamber effectively reduces the distance between electrodes and thereby reduces the internal resistance of the entire cell. With 0.6 V of applied voltage, the cuboid MEC had a columbic efficiency of 33.7%, much higher than that achieved in the H-shaped MEC test (ca. 15%) of comparable size. Filling the anode chamber with granular activated carbon further enhanced the columbic efficiency to 45%. The corresponding hydrogen conversion rate could reach 35%. (C) 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.