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ABSTRACt The x-ray standing wave technique has been employed in the study of the structure of underpotentially deposited (UPD) copper on an iodine covered platinum surface and of copper on a Au(100) single crystal electrode. For Cu UPD on Pt, surface coverage isotherms derived from both electrochemical and x-ray measurements are compared. The growth mode of the copper ad-layer appears to be strongly influenced by the electrode's surface morphology. For Cu UPD on Au(100) the coherence of the adlayeris strongly dependent on the mode of deposition. 4 • The process of underpotential deposition (UPD) of metals has been extensively I studied during the past two decades due to its theoretical and practical importance.[1] In S0 this process, submonolayer to monolayer(s) amounts of a metal can be electrodeposited M 4() on a foreign metal substrate in a quantifiable and reproducible fashion prior to bul