A morphological transformation of Ag electrode in 0.1 mol dm −3 KCl has been studied by in situ STM observation by controlling the electrode potential during a single electrochemical oxidation–reduction cycle (ORC) treatment in conjunction with SERS. Prior to the oxidation the Ag electrode surface has ellipsoidal structures. After the single oxidation treatment electrochemical formation of AgCl clusters and adatoms occurred at the edge of ellipsoidal structures. These species were stripped by the reduction processes. A single ORC treated Ag surface has roughness on an atomic scale due to the presence of residual AgCl clusters, adatoms, and adsorbed chloride ions. The ORC treated surface has roughness characteristics more suitable for observing the SERS effect than the electrochemically untreated surface.