The underpotential deposition (upd of silver and copper onto Au(111) can be performed sequentially to produce a composite monolayer with a laterally mixed structure. In situ atomic force microscopy (AFM) was used to follow the upd processes on Au(lll) under potential control in Ag+ and Cu2+ electrolytes. The first upd step in a sulfuric acid-based Ag+ electrolyte produced a partial monolayer of Ag atoms on Au(111). Cyclic voltammetry with this Au(111)/Ag upd sample in a Cu2+ electrolyte showed a upd peak in both the anodic and cathodic scans that was shifted by approximately -100 mV from that on bare Au(lll). X-ray photoelectron spectroscopy and in situ AFM measurements revealed that the upd peaks for Cu on Au(111)/Ag resulted from the reversible transition in surface structure between an open hexagonal structure of Ag atoms on Au(lll) and a (1 x 1) adlayer structure comprising both Ag and Cu atoms on the Au(lll) surface. In formation of the ii x 1) structure, the Cu atoms filled the open spaces on Au(lll) created by the Ag upd layer.