The underpotential deposition (upd) of Cd on Cu(111) single crystal electrodes was studied by cyclic voltammetry and UHV based spectroscopies. The structure of the Cd upd layer was identified as a (root 19 x root 19)R23.4 degrees structure with a Cd coverage of 9/19 monolayers with respect to Cu(111). The Cd-Cd distance within the layer is 3.71 Angstrom, much larger than in bulk Cd. A Cl layer is adsorbed on top of the Cd and stabilizes the open Cd structure.