Recently, extensive investigations have been carried out on the electrochemical and photoelectrochemical reductions of CO2 into hydrocarbons to convert renewable energies into chemical fuels. In these researches, aqueous solutions of bicarbonate salts, such as NaHCO3 and KHCO3, are widely used as the supporting electrolyte. However, the effects of HCO3- on the CO2 reduction have not been studied in detail. In this research, the effects of KHCO3 concentration on CO2 reduction were studied at different applied potentials on a Cu working electrode using a potentiostatic method. The concentration of dissolved CO2 ([CO2]) and ratio of [CO2]/[K+] in the KHCO3 solution after bubbling with CO2 were studied to determine the relationship between KHCO3 concentration and CO2 reduction faradaic efficiency. The results suggest that high [CO2]/[K+] ratio in solutions with low KHCO3 concentration is probably a key factor to promote CO2 reduction faradaic efficiency. A model for the effect of [CO2]/[K+] ratio on CO2 reduction efficiency was also proposed. (C) 2017 The Electrochemical Society. All rights reserved.