This communication compares two different multiple deposition routes of Pt on Au( 111), using irreversible adsorption of Pt precursor ions and selective adsorption of CO. A scanning tunneling microscopy study revealed that the conventional route, not utilizing CO, produced multiple-layered Pt cluster islands, while the CO route, employing CO, formed single-layered Pt islands exclusively. The role of CO selectively adsorbed on pre-existing Pt islands was to prevent additional irreversible adsorption of Pt precursor ions onto Pt islands. Cyclic voltammetric works disclosed that the CO and hydrogen coverages on single-layered Pt islands were higher than those on multiple-layered ones, and that the Pt islands on Au were more effective in adsorbing CO than hydrogen.