The effect of pH on the conversion of glucose to hydrogen by a mixed culture of fermentative bacteria was evaluated. At 36 degreesC, six hours hydraulic retention, over 90% of glucose was degraded at pH ranging 4.0-7.0, producing biogas and an effluent comprising mostly fatty acids. At the optimal pH of 5.5, the biogas comprised 64 +/- 2% of hydrogen with a yield of 2.1 +/- 0.1 mol-H-2/mol-glucose and a specific production rate of 4.6 +/- 0.4 1-H-2/(g-VSS day). The effluent was composed of acetate (15.3-34.1%) and butyrate (31.2-45.6%), plus smaller quantities of other volatile fatty acids and alcohols. The diversity of microbial communities increased with pH, based on 16S rDNA analysis by denaturing gradient gel electrophoresis (DGGE).