The adsorption of 3-mercaptopropanesulfonate (MPS) molecule on a Pt(111) single-crystal electrode and its effect on the deposition of Cu have been examined using in situ scanning tunneling microscopy (STM). MPS admolecules were irreversibly adsorbed in a largely disordered adlayer on bare Pt(111) in 0.1 M HClO4, irrespective of the presence of chloride, the concentration of MPS, and the applied potential. In comparison, the MPS admolecules readily formed a highly ordered molecular structure identified as (4x2 root 3)rect on Pt(111) precoated with a monolayer of Cu adatoms. The MPS admolecules were adsorbed upright on Pt(111). The cyclic voltammetric results show that the MPS adlayer on Pt(111) would inhibit Cu deposition because the addition of 10 mu M MPS to the electrolyte of 0.1 M HClO4+1 mM KCl+1 mM Cu(ClO4)(2) reduced the amount of the Cu deposit by half, even in the presence of chloride. The texture of the Cu deposit also varied with the surface state of the Pt(111) electrode as the Cu film grew in three-dimensional islands and smooth flakelike morphology on the MPS-modified and Cu-coated Pt(111) electrodes, respectively. In situ STM results indicated that the MPS admolecules stayed afloat rather than buried by the Cu deposit.