Sewage sludge microflora were anaerobically cultivated in a chemostat-type anaerobic bioreactor at temperatures of 30-55 degrees C, a pH of 7.1 and a hydraulic retention time of 12 h to determine the hydrogen production efficiency from xylose (20g-COD/L). It was demonstrated that hydrogen production of the enriched sewage sludge microflora (dominated by Clostridia species) was temperature-dependent in hydrogen gas content, hydrogen yield, hydrogen production rate (HPR) and specific HPR with values of 25.1-42.2% (v/v), 0.4-1.4 mol-H-2/mol-xylose, 0.06-0.24 mol-H-2/L-day and 0.02-0.10 mol-H-2/g-VSS-day, respectively, and the above values peaked when being operated at 50 degrees C. A transition temperature of 45 degrees C existed by having a lowest hydrogen production efficiency. Butyrate and ethanol were the major soluble metabolite products for thermophilic and mesophilic fermentation, respectively. The liquid metabolite concentration fractions and microbial community analysis indicate that the differences in hydrogen production efficiency between each tested temperature might relate to the shifts in metabolic pathway or microbial community. Thermodynamic analysis using HPR values and Arrhenius equation showed that the activation energy was 74.7 kJ/mol. Strategies based on temperature control for optimal hydrogen production from xylose using natural mixed cultures are proposed. (C) 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.