Lignocellulosic biomass such as agri-residues, agri-processing by-products, and energy crops do not compete with food and feed, and is considered to be the ideal renewable feedstocks for bio-fuel production. Gasification of biomass produces synthesis gas (syngas), a mixture primarily consisting of CO and H-2. The produced syngas can be converted to ethanol by anaerobic microbial catalysts especially acetogenic bacteria such as various clostridia species. One of the major drawbacks associated with syngas fermentation is the mass transfer limitation of these sparingly soluble gases in the aqueous phase. One way of addressing this issue is the improvement in reactor design to achieve a higher volumetric mass transfer coefficient (k(L)a). In this study, different reactor configurations such as a column diffuser, a 20-mu m bulb diffuser, gas sparger, gas sparger with mechanical mixing, air-lift reactor combined with a 20-mu m bulb diffuser, air-lift reactor combined with a single gas entry point, and a submerged composite hollow fiber membrane (CHFM) module were employed to examine the k(L)a values. The k(L)a values reported in this study ranged from 0.4 to 91.08 h(-1). The highest k(L)a of 91.08 h(-1) was obtained in the air-lift reactor combined with a 20-mu m bulb diffuser, whereas the reactor with the CHFM showed the lowest k(L)a of 0.4 h(-1). By considering both the k(L)a value and the statistical significance of each configuration, the air-lift reactor combined with a 20-mu m bulb diffuser was found to be the ideal reactor configuration for carbon monoxide mass transfer in an aqueous phase. (C) 2010 American Institute of Chemical Engineers Biotechnol. Prog., 26: 1616-1621, 2010