This study investigated the effects of electrolytes (CaCl2, K2HPO4, MgSO4, NaCl, and NH4Cl) on CO mass transfer and ethanol production in a HFMBR. The hollow fiber membranes (HFM) were found to generate tiny gas bubbles; the bubble coalescence was significantly suppressed in electrolyte solution. The volumetric gas-liquid mass transfer coefficients (k(L)a) increased up to 414% compared to the control. Saturated CO (C*) decreased as electrolyte concentrations increased. Overall, the maximum mass transfer rate (R-max) in electrolyte solution ranged from 106% to 339% of the value obtained in water. The electrolyte toxicity on cell growth was tested using Clostridium autoethanogenum. Most electrolytes, except for MgSO4, inhibited cell growth. The HFMBR operation using a medium containing 1% MgSO4 achieved 119% ethanol production compared to that without electrolytes. Finally, a kinetic simulation using the parameters got from the 1% MgSO4 medium predicted a higher ethanol production compared to the control.