To enhance hydrogen (H-2) production from glucose by Enterobacter aerogenes HU-101, two mutants, strains VP-1 and VP-2, with decreased alpha-acetolactate synthase activity, were isolated using the Voges-Proskauer (VP) test. In pH-uncontrolled batch culture, both mutants showed a lower 2,3-butanediol yield for the glucose consumed than that shown by the wild-type strain, although glucose remained in the medium after 12 h of culture. In the same cultures, compared to the H-2 yield of 0.80 mol/mol-glucose of the wild-type strain, strain VP-1 showed a high H-2 yield of 1.8 mol/mol-glucose with decreased lactate and increased succinate yields, while strain VP-2 showed an H-2 yield of 1.0 mol/mol-glucose with an increased lactate yield. Increasing the phosphate buffer concentration., which contributes to maintaining the pH in the medium, increased the glucose consumption by both strains. However, in a pH-controlled batch culture at neutral pH, the H-2 yield of strain VP-1 was as decreased to 1.2 mol/mol-glucose due to the accumulation of formate, an intermediate of the H-2-producing pathway, with the yield of H-2 plus formate being 1.7 mol/mol-glucose.