In this paper, the effect of Ni2+ concentration ranging from 0 to 50 mg/L on fermentative hydrogen production by mixed cultures was investigated in batch test. The results showed that at 35 degrees C and initial pH 7.0, Ni2+ was able to enhance the hydrogen production rate with increasing Ni2+ concentration from 0 to 0.2 mg/L, and enhance the hydrogen production potential and hydrogen yield with increasing Ni2+ concentration from 0 to 0.1 mg/L. The maximum hydrogen production potential of 288.6 mL and the maximum hydrogen yield of 296.1 mL/g glucose were obtained at the Ni2+ concentration of 0.1 mg/L. In all tests, the major soluble metabolites produced by mixed cultures were ethanol, acetic acid and butyric acid, without propionic acid. Ni2+ had little effect on the substrate degradation efficiency with increasing concentration from 0 to 50 mg/L. Ni2+ was able to enhance the biomass production yield with increasing Ni2+ concentration from 0 to 0.1 mg/L. The maximum biomass production yield of 232.5 mg/g glucose was obtained at the Ni2+ concentration of 0.1 mg/L. In all tests, the final pH after fermentative hydrogen production was lower than the initial pH. (C) 2008 Elsevier Ltd. All rights reserved.