Anaerobic sewage sludge acclimated with sucrose in a completely stirred tank reactor operating at 35degreesC was used as the seed (dominated by Clostridia species) in a batch experiment to investigate the concentration effects of carbonate and phosphate on biological hydrogen production. The Taguchi orthogonal array was used in the experimental design and a confirmation experiment was conducted. Experimental results indicated that the hydrogen production ability of the anaerobic microflora in sewage sludge was affected by NH4HCO3, Na2HPO4 and Na2CO3 with Na2HPO4 being the most significant supplement. Na2HPO4 affected the hydrogen production in a concentration-dependent way with 600 mg/l the optimal concentration. The hydrogen production rate was 1.9 times enhanced, compared with an acidogenic nutrient formulation (Proc. Soc. Civ. Engrs. 325 (1982) 61), when a proper carbonate and phosphate concentration formulation was used. The enhancement was due to a shortening of the microflora lag-time. Based on the enhancement abilities on hydrogen content and hydrogen production rate, adding phosphate instead of carbonate as a buffering capacity supplement might be a useful strategy for optimal hydrogen production operations with anaerobic sewage cultures. (C) 2003 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.