The electrochemical deposition of platinum on an Au(111) single-crystal electrode in acidic solutions containing H₂PtCl₆ was studied using an electrochemical scanning tunneling microscope (STM) and electrochemical quartz crystal microbalance (EQCM). The STM investigation showed an ordered adlayer of PtCl₆^2- on the electrode surface during the electrochemical deposition of platinum and a Pt(111)-(1×1) structure on the electrode surface after the electrode was rinsed with a Pt complex-free solution. Formations of the Pt(111) bulk phase and surface structure of Pt(111)-(1×1) were confirmed by X-ray diffraction (XRD) and the underpotential deposition of copper and hydrogen, respectively.