Anaerobic sewage sludge acclimated with sucrose was used as the seed in a batch experiment to investigate the carbon/nitrogen (C/N)-ratio effects on biological hydrogen production from sucrose. Experimental results indicated that the hydrogen production ability of the anaerobic microflora. (dominated by Clostridium pasteurianum) in the sewage sludge was dependent on the influent C/N-ratio. At a C/N-ratio of 47, the hydrogen productivity and hydrogen production rate reached 4.8 mol-H-2/mol-sucrose and 270 mmol-H-2/L-day, respectively. This increased by 500% and 80%, respectively, compared with the blank. Proper C/N-ratio on hydrogen production enhancement was accomplished by shifting the metabolic pathway. Strategies for optimal hydrogen production are also proposed. (C) 2003 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.