The adsorption of CO on copper in buffered phosphate solutions occurs at two types of binding sites, linearly bonded CO, COL and bridge bonded CO, COB. A competing. adsorption reaction between CO and pre-adsorbed anions occurs and the removal of anions from the surface is crucial for the adsorption of CO to occur. The appearance of an adsorbed CO and its preference for an adsorption site on copper are dependent on the magnitude and duration of the applied polarisation potential, the concentration of CO in solution, CO surface coverage and temperature. COL appears immediately after the surface is polarised at a high,negative potential and continues to be observed in the potential range -1.5 to -1.0 V versus saturated calomel electrode. On the other hand, COB becomes the dominant species as the electrode is polarised for prolonged periods and is stable over a wide potential window, from -1.5 to -0.1 V. At potentials more positive than -0.1 V, the oxidation of the copper surface is accompanied by the formation of Cu(I) carbonyl, The CO in the carbonyl originates from COB.