As a novel and electrochemical method, formate, anion is effectively formed by cathodic electrolysis of a small amount (ca. 0.5 mol l(-1)) of water in the presence of carbon monoxide in DMF containing a tetrabutylammonium salt. The electrogenerated hydroxide ion is shown to be nucleophilic toward CO at atmospheric pressure and room temperature. Use of deuterium oxide instead of water showed a high incorporation of D in the product formate, revealing some accompanying reaction paths such as hydrogen exchange with OD- and direct reduction of the ammonium salt.