Batch experiments were conducted to convert starch in wastewater into hydrogen at 55 degrees C at various wastewater pH (4.0-9.0) and starch concentrations (9.2-36.6 g/l). The maximum hydrogen yield of 92 ml/g of starch added (17% of the theoretical value) was found at wastewater pH 6.0, and the maximum specific hydrogen production rate of 365 ml/(g-VSS.d) was at wastewater pH 7.0. The methane-free biogas contained up to 60% of hydrogen. The mixed liquor was composed mostly of acetate (40.2-53.4%) and butyrate (26.0-40.9%). Phylogenetic analysis based on 16S rDNA sequences of the 72 clones developed from the sludge at pH 6.0 shows that 85.7% of the clones were closely affiliated with genus Thermoanaerobacterium in family Thermoanaerobacteriaceae; the remaining 14.3% were with an uncultured Saccharococcus sp. clone ETV-T2.