Abstract Anaerobic bacteria may be utilized in the conversion of CO, CO 2 and H 2 in synthesis gas to products such as methane, acetate, ethanol and butanol. Bioreactors for these fermentations are mass transfer limited due to very low gas solubilities. Several reactor designs have been examined for these conversions including batch, continuous stirred tank and bubble column reactors. This paper presents laboratory results for these reactor systems using the strict anaerobe Peptostreptococcus productus as a model organism. Results comparing the performance of these reactors and showing the effects of increased mass transfer and total pressure on system performance are presented are discussed.