In order to investigate the copper underpotential deposition (upd) on gold single crystals, X-ray surface diffraction (XRSD) measurements were performed under in-situ conditions. After formation of a complete Cu upd layer, Cu was found to adsorb in a commensurate p(1 x 1) structure in fourfold hollow sites with a vertical distance of 1.4 Angstrom from the Au(100) surface. The coverage obtained by XRSD was (65 + 8)%. The structure of the Cu upd layer was studied as a function of applied potential and of the Cu-ion concentration in solution. The Cu-ion concentration did not affect the structure of the Cu upd layer. As a function of the applied potential, it was possible to investigate the formation of the Cu upd layer by XRSD. From a comparison between the XRSD results and the cyclic voltammetry, the formation of an ionic adlayer is inferred for the Cu upd on Au(100) in perchloric acid.