Fermentative H-2 production in an upflow anaerobic sludge blanket reactor (UASB) at various pH values was investigated in this study. Experimental results show that the H-2 partial pressure in biogas, H-2 production rate and H-2 yield were all pH-dependent, in the range of 0.25-0.52 atm, 42-145 ml-H-2 I-1 h(-1) and 0.47 to 1.61 mol-H-2 mol-glucose(-1), respectively. The maximum pH for the H-2 partial pressure was observed at pH 7.50. However, the optimum H-2 production rate and H-2 yield were observed at pH 6.50-7.50. In this UASB reactor, acetate, propionate, butyrate, i-butyrate, valerate, caporate and ethanol were present in the effluent as main aqueous products, and the dominant fermentation was butyrate-type at various pHs. The metabolic pathways and thermodynamics of H-2 production were also analyzed. Both H-2 production performance and fermentation pathways in this H-2-producing UASB reactor were significantly affected by the pH value. (C) 2007 Elsevier Ltd. All rights reserved.