Comparisons were made between the performance of two upflow anaerobic biohydrogen-producing reactors. One reactor (R1) was seeded with methanogenic sludge, but was operated at low pH levels, whereas another (R2) was seeded with heat-pretreated sludge. Experiment results showed that H-2 partial pressure in biogas, H-2 production rate and H-2 yield were pH-dependent in the two reactors. The optimum pH for H-2 partial pressure was observed at pH 3.10 for R1 and 4.00 for R2, respectively. The optimum pH for maximum H-2 production rate and H-2 yield was 4.00 for both R1 and R2. Acetate, propionate, butyrate, i-butyrate, valerate, caproate and ethanol were main aqueous products and their distribution was also significantly influenced by pH. For R1, caproate was dominant at pH < 4.00 and the butyrate-type fermentation predominated at pH 4.5-6.0. For R2, butyrate was dominant at pH > 3.60, while the ethanol-type fermentation was important at pH < 3.60. With a thermodynamic analysis, possible metabolic pathways for the formation of caproate and valerate in both reactors were suggested. (C) 2006 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.