CO is reversibly adsorbed on a Cu electrode with simultaneous charge transfer, as observed by voltammetric measurements. In-situ FTIR measurements were conducted with a Cu(100) electrode in a phosphate buffer solution (pH 6.8) and a polycrystalline Cu electrode in a carbonate buffer solution (pH 10.3). With increase of the negative potential, the infrared absorption band of anions (phosphate anion and CO32-) diminishes at the potential of the charge transfer, whereas that of the adsorbed CO increases. Specifically adsorbed anions will remain still on the electrode surface below the potential of zero charge (prc) to some extent. The apparent charge transfer is observed at a potential below the pzc, where CO is adsorbed, displacing specifically adsorbed anions.