CO is adsorbed on Cu electrode with a simultaneous transfer of charge. The apparent charge transfer is derived from displacement of anions specifically adsorbed on the electrode during the adsorption of CO. This paper presents the results of voltammetric measurements of the adsorption of CO on single crystal electrodes of Cu[110] zone in phosphate buffer solution (pH 6.8)at 0 degrees C. The profile of the charge transfer wave depends significantly on the crystal orientation of Cu single crystals. The potential of the charge transfer is well correlated with the work functions and the potential of zero charge of the Cu single crystals. We propose a model that H2PO4 is specifically adsorbed on (100) steps of Cu[110] zone crystal electrodes occupying 3 to 4 atomic sites.