Our newly developed carrier-induced granular sludge bed (CIGSB) bioreactor was shown to be very effective in hydrogen production. However, since mechanical agitation was not employed to enable sludge granulation, the CIGSB system might encounter problems with poor mass transfer efficiency during prolonged operations. This work was undertaken to improve mixing efficiency of CIGSB for better biomass-substrate contact by adjusting the height to diameter (H/D) ratios of the reactor and by implementing appropriate agitation device. Three H/D ratios (4, 8, and 12) resulting in liquid upflow velocities (v(up)) of 0.057-1.32 m/h were examined as the CIGSB reactor was carried out at a descending hydraulic retention time (HRT) from 4 to 0.5 h. The results show that decreasing HRT resulted in increases in the H-2 production rate, regardless of the H/D ratios. Reactors with a H/D ratio of 8 gave better H-2 production performance with a H-2 production rate of 6.87 l/h/l and a H-2 yield of 3.88 mol H-2/mol sucrose, suggesting that the effectiveness of H-2 production in the CIGSB system can be enhanced by using a proper vup and physical configuration of the reactor. Supply of additional mechanical agitation for CIGSB reactor (H/D = 12) alleviated the phenomena of sludge piston floatation, leading to further increases in the H-2 production rate and H-2 yield to 9.31 l/h/l and 4.02 mol H-2/mol sucrose, respectively. The major soluble metabolite was butyric acid, followed by acetic acid, propionic acid, and ethanol. The former two accounted for nearly 67-76% of total soluble microbial products, indicating the presence of favorable pathways in the CIGSB culture from the aspect of H-2-producing metabolism. (c) 2006 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.