An efficient syngas fermentation bioreactor provides a mass transfer capability that matches the intrinsic kinetics of the microorganism to obtain high gas conversion efficiency and productivity. In this study, mass transfer and gas utilization efficiencies of a trickle bed reactor during syngas fermentation by Clostridium ragsdalei were evaluated at various gas and liquid flow rates. Fermentations were performed using a syngas mixture of 38% CO, 28.5% CO2, 28.5% H-2 and 5% N-2, by volume. Results showed that increasing the gas flow rate from 2.3 to 4.6 sccm increased the CO uptake rate by 76% and decreased the H-2 uptake rate by 51% up to Run R6. Biofilm formation after R6 increased cells activity with over threefold increase in H-2 uptake rate. At 1662 h, the final ethanol and acetic acid concentrations were 5.7 and 12.3 g/L, respectively, at 200 ml/min of liquid flow rate and 4.6 sccm gas flow rate. (C) 2016 Elsevier Ltd. All rights reserved.