Great interest has emerged in biological CO2-fixing processes in the context of current climate change discussions. One example for such a process is the hydrogenotrophic production of acetic acid by anaerobic microorganisms. Acetogenic microorganisms make use of carbon dioxide in the presence of hydrogen to produce acetic acid and biomass. In order to establish a process for the hydrogenotrophic production of acetic acid, the formation of acetate by Acetobacterium woodii was studied in a batch-operated stirred-tank bioreactor at different hydrogen partial pressures (pH(2)) in the gas phase. The volumetric productivity of the batch processes increased with increasing hydrogen partial pressure. A maximum of the volumetric productivity of 7.4 g(acetate) L-1 day(-1) was measured at a pH(2) of 1,700 mbar. At this pH(2) a final acetate concentration of 44 g L-1 was measured after a process time of 11 days, if the pH was controlled at pH 7.0 (average cell density of 1.1 g L-1 cell dry weight). The maximum cell specific actetate productivity was 6.9 g(acetate) g(cdw)(-1) day(-1) under hydrogenotrophic conditions. Biotechnol. Bioeng. 2011;108: 470-474. (C) 2010 Wiley Periodicals, Inc.