In situ scanning tunneling microscopy (STM) was used to examine the ordered microstructures of platinum (Pt) deposited on Au(111) substrate in aqueous Na₂PtCl₆ solution saturated with carbon monoxide (CO) without potential control at room temperature. PtCl₆^2- complexes were reduced by CO molecules and deposited on Au(111) in a hexagonal array without potential control. At the same time, we introduced electron spectroscopy for chemical analysis (ESCA) to prove the Pt adatoms were reduced to metallic state after the CO was oxidized in air. According to the results of CV and STM, the coverage of Pt on Au(111) substrate increased sharply with the time passed initially and finally had the value of 0.87monolayer (ML) after 200min dosing. After 5min dosing, the adsorption of CO molecules first mobilized and rearranged Pt adatoms on Au(111) from disordered Pt aggregates to 0.9 nm wide tortuous stripes, and finally to patches of ordered Pt adatoms at 0.1 V (versus reversible hydrogen electrode) in 0.1M H₂SO₄ saturated with CO. However, the Pt adlayer seem to be too stable to be observed any change with time after 10min dosing. Deposition of Pt in the presence of CO ceased with a monatomic Pt film capped with CO molecules arranged in a (2 × 2) array in both with and without potential control. The CO monolayer adsorbed on Pt/Au(111) was stripped off at 0.96 V in 0.1 M H₂SO₄, as compared with 0.83 V observed with Pt(111). The E_1/2 potential for oxygen reduction reaction (ORR) observed with the Pt-modified Au(111) electrode was 0.81 V in 0.1M sulfuric acid, which is 0.16 V more positive than that of Pt(111). The microstructures of the as-prepared Pt monolayer on Au(111) changed with potential cycling between 0.05 and 1.0 V.