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 PtCl62- on the electrode surface during the electrochemical deposition of platinum and a Pt(111)-(1x1) structure an 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)-(1x1) were confirmed by X-ray diffraction (XRD) and the underpotential deposition of copper and hydrogen, respectively.