The results presented in this study show definitely that copper deposition in perchloric acid is a very slow, kinetically controlled process. In nearly halide free supporting electrolytes du appeared to be deposited at underpotential in metallic islands (or ＇patches＇) having the platinum lattice constant. The presence of Br- anions significantly enhances the kinetics of the Cu UPD on the Pt(100) electrode. The total amount of Cu deposited at underpotential is ca. 1 ML (1 ML = 1 Cu adatom per Pt atom). The results obtained from the surface X-ray diffraction measurements showed the formation of an ordered structure of Pr into a c(2 x 2) unit cell on the top of a p(1 x 1) pseudomorphic layer of Cu. In a solution free of Cu no ordered structure of Pr has been found on Pt(100). RRDE and SXS results indicate beyond any doubt that the surface coverage by Br undergoes only negligible changes upon the deposition of Cu even up to a nominal monolayer. We propose, therefore, that the mechanism of Cu UPD occurs by displacement of bromine adatoms from the Pt surface by Cu adatoms through a ＇turn-over＇ process in which Cu is sandwiched between the Pt surface and the Brad overlayer, i.e. a Pt(100)-Cu-Br bi-layer structure.