The hydraulic retention time (HRT) effects on fermentative hydrogen production from sucrose at high sulfate concentration of 3 g-SO(4)(2-)/l were studied using enriched mixed-microflora in a continuously fed reactor. The tested HRTs and organic loading rate ranged from 10 to 2 h and 48 to 240 g-COD/l-day, respectively, and the operating pH was 5.5. The experimental results indicate that hydrogen production could not be inhibited under high sulfate concentration and the efficiency was HRT-dependent with a short HRT of 4h efficiently enhanced hydrogen production. At this HRT the biogas production rate and hydrogen gas content peaked with the hydrogen yield, hydrogen production rate and specific hydrogen production rate of 4.70 mol-H(2)/mol-sucrose, 874 mmol-H(2)/l-day and 432 mmol-H(2)/g-VSS-day, respectively. These values were 50%, 80% and 300%, respectively, higher than those reported for 12 h HRT at the same sulfate concentration. The metabolite concentration fractions were butyrate 77.3%, acetate 15.6%, ethanol 4.4% and propionate 2.0% and changed to 55%, 27.3%, 11.2% and 6.5%, respectively, at HRT 2 h. Therefore, intimate HRT control is important to obtain efficient hydrogen production. Based on a biological growth comparison, pH 5.5 was considered to be the optimal value for operating a hydrogen-producing fermenter fed on sulfate-rich substrate. (c) 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.