The work exploring the stoichiometry of Pt deposition via surface-limited redox replacement of the underpotentially deposited (UPD) Cu monolayer on Au(111) is presented. The Cu UPD monolayer is formed from 10(-3) M Cu2+ + 0.1 M HClO4 solution, whereas the Pt deposition via surface-limited redox replacement reaction is carried out in 10(-3) M {PtCl6}(2-) + 0.1 M HClO4 solution at open-circuit potential. Our results indicate that the Pt submonolayers have two-dimensional morphology and linear dependence of their coverage on the amount (coverage) of the replaced Cu UPD monolayers. Our analysis shows that the oxidation state of Cu during redox replacement reaction is 1+, suggesting that four Cu UPD adatoms are replaced by each deposited Pt adatom. This work stresses the general importance of the anions, determining the stoichiometry of metal deposition reaction via surface-limited redox replacement of the UPD monolayers. (c) 2010 The Electrochemical Society. [DOI: 10.1149/1.3490416] All rights reserved.