Continuous production of hydrogen from the anaerobic acidogenesis of a high-strength rice winery wastewater by a mixed bacterial flora was demonstrated. The experiment was conducted in a 3.0-1 upflow reactor to investigate individual effects of hydraulic retention time (HRT) (2-24 h), chemical oxygen demand (COD) concentration in wastewater (14-36 g COD/1), pH (4.5-6.0) and temperature (20-55degreesC) on bio-hydrogen production from the wastewater. The biogas produced under all test conditions was composed of mostly hydrogen (53-61%) and carbon dioxide (37-45%), but contained no detectable methane. Specific hydrogen production rate increased with wastewater concentration and temperature, but with a decrease in HRT. An optimum hydrogen production rate of 9.331 H-2/g VSS d was achieved at an HRT of 2 h, COD of 34 g/l, pH 5.5 and 55degreesC. The hydrogen yield was in the range of 1.37-2.14 mol/mol-hexose. In addition to acetate, propionate and butyrate, ethanol was also present in the effluent as an aqueous product. The distribution of these compounds in the effluent was more sensitive to wastewater concentration, pH and temperature, but was less sensitive to HRT. This upflow reactor was shown to be a promising biosystem for hydrogen production from high-strength wastewaters by mixed anaerobic cultures. (C) 2002 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved.