A base-enriched anaerobic mixed microflora was used for hydrogen fermentation from sucrose in a laboratory scale model completely stirred tank bioreactor operating at 35 degrees C. The purpose of the study was to determine the microflora hydrogenic activity and its effects from the change of hydraulic retention time (HRT, 2-12 h). The experimental results indicate that base-enriched mixed microflora could be used as the seed for efficient hydrogen fermentation. The fermenter could operate stably for 250 days at a HRT of 12 h. Hydrogen gas content, hydrogen productivity and hydrogen production rate were HRT-dependent and their values ranged 38.7-45.9%, 0.9-3.5 mol H-2/mol sucrose and 263-408 mmol H-2/L day, respectively, with a HRT of 4 h having peak hydrogen production. The biomass activity was also HRT-dependent with each gram of biomass producing 65-145 mmol H-2/day. The DGGE analysis shows that the microbial species shifted during the HRT-reduction operation but Clostridium ramosum was dominant. Those hydrogen productivity values were comparable to the hydrogen production using pure cultures, other mixed microflora or other reactor systems. The major liquid fermentation products were ethanol, acetic, propionic and butyric acids; their concentrations were also HRT-dependent. Strategies based on these results for optimal hydrogen production were proposed. (c) 2005 Elsevier Ltd. All rights reserved.