The underpotential deposition (upd) of Ag on a Au(111) electrode in aqueous sulphuric acid has been studied by electrochemical scanning tunneling microscopy (ECSTM). The Au(111) surface morphology is substantially altered during repeated cycles in the Ag upd region. Prior to Ag upd, the surface consists of large, flat terraces (50-200 nm in width) with monatomic and diatomic steps. After several cycles of Ag deposition and dissolution, the surface is characterised by the appearance and growth of a high density of monolayer deep 'pits.' As cycling is continued, the pits expand rapidly to leave isolated Au islands accompanied by receding terrace edges. This process is followed by a slow lateral growth of Au islands which apparently fuse with terrace edges or aggregate with each other to form extended, atomically Bat terraces. The surface topography is dominated by straight-edged, narrow terraces (15-50 Mm in width) running parallel to or crossing each other to form a mesh, in which the terrace edges meet at an angle of 120 or 60 degrees. characteristic of a (111) close-packed plane. The slow transformation of a high quality Au(111) surface in comparison to the faster transformation of a defect rich surface suggests that the kinetics of the transition process are related to the initial number of surface defects. The surface morphological changes accompanying Ag upd cycling are reflected in the cyclic voltammograms via dramatic changes in both peak shape and potential. The effect of variation of the supporting anion on this transition has also been investigated. Repetitive cycling of Ag upd on the Au(111) electrode in the presence of perchlorate anions results in similar alterations in voltammetric peak shape and potential. This weakly adsorbing anion appears to enhance the rate of structural transformation of the Au(111) surface.