In situ scanning tunneling microscopy (STM) was used to examine the microstructures of platinum (Pt) deposited on ordered Au(111) substrate in aqueous Na2PtCl6 solution saturated with carbon monoxide (CO) with and without potential control at room temperature. PtCl62- complexes were reduced to Pt atoms by CO molecules and deposited on Au(111) in a hexagonal array without potential control. 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 then to patches of ordered Pt adatoms at 0.1 V (versus reversible hydrogen electrode). Electrodeposition of Pt in the presence of CO ceased with a monatomic Pt film capped with CO molecules arranged in a (2 × 2) array at 0.1 V. 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 E1/2 potential for oxygen reduction reaction (ORR) observed with the Pt-modified Au(111) electrode was 0.81 V in 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.