The underpotential deposition of Pb was studied on Cu(111) single crystal surfaces prepared both by a novel electropolishing procedure and by sputtering/annealing in ultrahigh vacuum. Identical,results were found with both methods. Pb atoms are deposited underpotentially on Cu(111) into a compact nonrotated hexagonal overlayer. The measured Pb coverage at saturation is 53% with respect to the Cu(111) substrate and is identical to the packing density of the (111) plane of bulk Pb. The presence of Cl- in the supporting electrolyte has a strong effect on the potential region where deposition/stripping occurs and on the reversibility of the reaction.