A membrane bioreactor (MBR) fabricated by connecting a hollow-fiber microfiltration membrane module with a continuous flow stirred tank reactor (CSTR) was used to enhance H-2 production through high-dilution rate operations. Three different carbon substrates (glucose, sucrose, and fructose) were examined for their effectiveness in H-2 production with a mixed microflora. The results show that in CSTR operation, cell washout occurred at a hydraulic retention time (HRT) of 2-4h. Using MBR could avoid cell washout, leading to a substantial increase in both H-2 production rate (HPR) and biomass concentration. The MBR system was very effective in retaining biomass within the reactor as the system can be stably operated at an extremely low HRT of I h with an optimal steady-state HPR of 1.48, 2.07, and 2.75 l/h/l, respectively, for using glucose, sucrose, and fructose as the sole carbon source. Meanwhile, despite operation at a high dilution rate (i.e., HRT = I h), the H-2 yield (HY) could be maintained at a high level of 1.27, 1.39, and 1.36 mol H-2/mol hexose, for glucose, sucrose, and fructose, respectively. Irrespective of the bioreactor mode (CSTR or MBR), the HPR tended to decrease in the order of fructose > sucrose > glucose. Thus, fructose seems to be the most efficient carbon substrate for H-2 production with the H-2-producing mixed culture used in this work. Butyrate (HBu) and acetate (HAc) (especially, HBu) were the major soluble metabolites in all cases, contributing to 70-85% of total soluble microbial product (SMP). The HPR and HY could be estimated based on stoichiometric correlation between formation of soluble metabolites (i.e., HBu, HAc, and propionate) and H-2 production. The estimated values are in good agreement with the experimental results. (C) 2006 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.