Adsorption of Cl- at the Au(lll) electrode in the presence of underpotentially deposited copper adatoms is described. It is shown that the underpotential deposition of copper promotes adsorption of Cl-, and at high or intermediate underpotentials a mixed overlayer which consists of copper adatoms and coadsorbed Cl- ions is formed. At intermediate underpotentials, the surface concentration of Cl- in the mixed overlayer amounts to 8x10(14) ions cm(-2) which is only 10% less than the concentration of Cl- in the close-packed monolayer of Cl-. This surface concentration of Cl- is independent of the concentration of Cl- and Cu2+ in, the bulk. In solutions of low Cl- or Cu2+ concentrations and at low underpotentials the surface concentration of Cl- decreases when the copper coverage approaches a monolayer. In solutions of high copper and chloride concentrations and low underpotentials the surface concentration of Cl- increases above the Value of 8 x 10(14) ions cm(-2). The increase in the surface concentration of Cl- is observed when the copper coverage exceeds one monolayer. Therefore it may suggest formation of copper clusters or a trilayer.