This study compares the power demand and gas-liquid volumetric mass transfer coefficient, k(L)a, in a stirred tank reactor (STR) (T = 0.211 m) using different impeller designs and schemes in a carbon monoxide-water system, which is applicable to synthesis gas ( syngas) fermentation. Eleven different impeller schemes were tested over a range of operating conditions typically associated with the "after large cavity" region (ALC) of a Rushton-type turbine ( D/T = 0.35). It is found that the dual Rushton-type impeller scheme exhibits the highest volumetric mass transfer rates for all operating conditions; however, it also displays the lowest mass transfer performance (defined as the volumetric mass transfer coefficient per unit power input) for all conditions due to its high power consumption. Dual impeller schemes with an axial flow impeller as the top impeller show improved mass transfer rates without dramatic increases in power draw. At high gas flow rates, dual impeller schemes with a lower concave impeller have kLa values similar to those of the Rushton-type dual impeller schemes but show improved mass transfer performance. It is believed that the mass transfer performance can be further enhanced for the bottom concave impeller schemes by operating at conditions beyond the ALC region defined for Rushton-type impellers because the concave impeller can handle higher gas flow rates prior to flooding.