The cation of the supporting electrolyte was found to play an important role in the electrochemical reduction of highly concentrated CO2 in a CO2 + methanol medium. Electroreduction of CO2 with tetrabutylammonium (TBA) salts yielded CO as the main product, while methyl formate was predominantly formed when lithium salts were used as supporting electrolytes. The latter supporting electrolytes showed a higher overvoltage than the former. When TBA salt was used, the reduction of CO2 was catalysed by TBA ion to yield CO2-.. This intermediate may be stabilized by forming an ion pair, {TBA(+)-CO2-.}, or by being adsorbed on the electrode surface as CO2ad-.. Then CO2-. reacts with CO2 to produce CO. The hydrophobic atmosphere at the electrode provided by TBA ion may be adequate for CO production. Lithium ion, on the other hand, suppressed the reduction of CO2.