Copper underpotential deposition (UPD) on a conductive hydrous ruthenium oxide (RuOxHy) surface was studied by progressive cyclic voltammetry and X-ray photoelectron spectroscopy (XPS). Cu UPD on an electrochemically prepared RuOxHy surface started at 0.15 V in a 2 mM CuSO4 solution and reached a coverage plateau of ca. 0.9 monolayer (ML) beyond -0.05 V (vs. Ag/AgCl). XPS data confirmed that Cu deposits underpotentially on RuOxHy surface. The anodic polarization potential determines the chemical states and affects the Cu UPD/bulk deposition of RuOxHy electrode. We observed close to a 55% increase of underpotential shift for Cu UPD on RuOxHy (ca. 170 mV) in comparison to Ru surface (ca. 110 mV). The results suggest that interfacial binding of the first ML of Cu on RuOxHy is stronger than Ru by 12 kJ mol(-1). Possible applications of Cu UPD to Ru-based plateable seedless Cu diffusion barrier are discussed. (C) 2004 The Electrochemical Society.