The electrodeposition of Cu on Ru(0001) and polycrystalline Ru was studied in a combined ultrahigh vacuum/electrocliemisti-y system (UHVEC) in 0.1 M HC10(4). The Cu(C10(4))(2) concentrations were varied from 0.005 M to 0.0005 M for deposition on Ru(000 1), while a solution of 0.05M H2SO4/0.005 M CuSO4/0.001 M NaCl was used for deposition on Ru(poly). Cyclic voltammograms show well-defined Cu underpotential (UPD) and overpotential deposition (OPD) peaks. X-ray photoelectron spectroscopy (XPS) was used to determine surface composition. Photoelectron spectra of Ru electrodes emersed from perchloric acid solution at cathodic potentials show that C10(4)(-) dissociates into adsorbed Cl- and C10(x)(-) species. The deposited Cu film consists of a bottom layer of Cu(I) and a top layer of insoluble Cu(II). In contrast, only Cu(0) is obtained when Cu is deposited on polycrystalline Ru in a solution consisting of H2SO4, CuSO4 and NaCl, indicating that the formation of Cu(II) and Cu(I) involves both Cl- and C104- interactions with the deposited Cu. A pre-adsorbed layer of iodine on the Ru(0001) surface inhibits perchlorate dissociation and promotes the deposition of Cu(0) in the perchlorate bath. XPS depth profile analysis demonstrates that the iodine monolayer "floats" on top of the deposited film, in agreement with previous results, effectively protecting the Cu film from air oxidation. (c) 2005 Elsevier B.V. All rights reserved.