The pH is one of the most important parameters for producing hydrogen in biological processes. The major effect of low pH is the inhibition of the methanogenic activity in anaerobic biological processes for the production of hydrogen and, in general, it is considered that a pH below 5 can inhibit the methanogenic activity. In a semi-continuous reactor for the production of hydrogen, the pH was maintained at 4.5 +/- 0.2 by fixing the influent alkalinity to 1000 mg/l as CaCO3. The primary fermentation product in this pH range was butyrate. The rate of hydrogen production stabilized at 35 5 ml/g Glu-day after 35 days. The yield efficiency of hydrogen was relatively low due to the methanogenic activity. The methanogenic activity originated from the hydrogen utilizing methanogen, and was inhibited by the addition of nitrate. At pH 4.3, the butyrate type of fermentation was changed to the butanol type and, simultaneously, the production of hydrogen stopped. (C) 2003 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.