In our previous work, we demonstrated that biomass-generated producer gas can be converted to ethanol and acetic acid using a microbial catalyst Clostridium carboxidivorans P7(T). Results showed that the producer gas (1) induced cell dormancy, (2) inhibited H-2 consumption, and (3) affected the acetic acid/ethanol product distribution. Results of this work showed that tars were the likely cause of cell dormancy and product redistribution and that the addition of a 0.025 mu m filter in the gas cleanup negated the effects of tars. C carboxidivorans P7(T) can adapt to the tars (i.e. grow) only after prolonged exposure. Nitric oxide, present in the producer gas at 150 ppm, is an inhibitor of the hydrogenase enzyme involved in H2 consumption. We conclude that significant conditioning of the producer gas will be required for the successful coupling of biomass-generated producer gas with fermentation to produce ethanol and acetic acid. (c) 2006 Elsevier Ltd. All rights reserved.