Ethanol can be produced from biomass feedstocks or municipal solid wastes (MSW) using a hybrid thermochemical-biochemical conversion process called gasification-fermentation. In the gasification-fermentation process, biomass is gasified and converted to synthesis gas (syngas; primarily CO, CO2 and H2). The advantage of gasification-fermentation over the saccharification-fermentation process is that gasification utilizes cellulose, lignin and hemicellulose components of the biomass, leading to a greater potential for high carbon conversion efficiency and high ethanol yield from the same amount of biomass. Syngas fermentation rates are subject to gas-liquid mass transfer limitations since CO and H2 have low solubilities in the fermentation broth. Successful syngas fermentation balances the kinetic capacity of the cell inventory with CO and H2 supply that is set by the mass transfer characteristics of the reactor used. Continuously stirred tank reactor (CSTR), trickle-bed reactor (TBR) and hollow fiber membrane reactor (HFR) were compared for syngas fermentation with regards to gas utilization (including mass transfer assessment), cell growth, and product formation. The volumetric mass transfer coefficient (KtotA/VL) for O2 in the three reactors was measured. The HFR provided the largest KtotA/VL followed by the TBR. The mass transfer characteristics in each reactor were affected by agitation speed, and gas and liquid flow rates. Correlations for KtotA/VL for each reactor were developed to predict mass transfer during syngas fermentation by Clostridium ragsdalei to produce ethanol and acetic acid. Although scale-up issues are different for each reactor, the comparison of the three reactors provides guidance as to which reactor(s) should be evaluated for scale-up, especially if one reactor significantly outperforms the others regarding ethanol yield and productivity and syngas utilization efficiency. Further, insight gained from the analysis of the three reactors can be extended to other fermentation processes.