The influence of the concentration of chloride ions on the underpotential deposition of copper onto Pt(111) has been studied by AES, LEED and RHEED. The results demonstrate that the arrangements of the superlattices of (2 x root 3), ＇(4 x 4)＇, c(11 x root 3) and ((8)(1)(1)(2)), real space structures of the corresponding LEED/RHEED patterns are), and their packing density on the copper covered Pt(111) are clearly determined by the Cl- concentration. derived from a comparison of the LEED spot intensities with those obtained by kinematic calculations. Thus the superstructures are interpreted as ordered Cl adlayers on top of a pseudomorphic-grown Cu UPD layer. A compressed copper UPD adlayer with a Cu-Cu distance of 0.259 nm determined by EXAFS could not be observed by RHEED.