The Cu UPD on different Ag substrates was investigated by cyclic voltammetry, potential step and X-ray photoelectron spectroscopy (XPS) experiments. Although it is known that a Cu UPD fails on bulk silver substrates, it could be proved that on silver monolayers copper ions are reduced in the underpotential region forming an Au-Ag-Cu sandwich structure. This process occurs only in absence of anions and thymine which are specifically adsorbed on the silver substrate. In this sense the Cu UPD on silver monolayers behaves completely different compared to the Cu UPD on Au(1 1 1) for which the process is supported only in presence of specifically adsorbed anions or thymine. (C) 2008 Elsevier B.V. All rights reserved.