In situ scanning tunneling microscopy (STM) was used to examine the spatial structure of adsorbed 3-mercaptopropanesulfonate (MPS) molecules on a Pt(111) electrode in 0.1 M KClO4 + 1 mM HCl + 10-7 M MPS (pH 3). Two ordered MPS structures, Pt(111)-(2 × 2) (θ = 0.25) and (̄3 × ̄3)R30° (θ = 0.33) structures were observed at -0.25 V (vs Ag/AgCl). The former (latter) was more important at more negative (positive) potentials. These MPS structures became a disordered adlayer at E > 0.1 V. These restructuring events could result from a progressive increase of the surface coverage of MPS with potential. Shifting the potential negatively could restore the ordered structures of (̄3 × ̄3)R30° and (2 × 2), but the rather strong Pt-MPS made it difficult for MPS admolecules to desorb from the Pt(111) electrode. By contrast, the MPS adlayer seen in 0.1 M HClO4 was always disordered, regardless of the potential of Pt(111) electrode. (Tu et al., J. Electrochem. Soc.2010, 157, D206.) It is reasonable to state that potential control, pH, and/or countercations to the sulfonate group of the MPS admolecule could be important in guiding the adsorption of MPS molecules on Pt(111) electrode. Strongly adsorbed MPS molecules on the Pt(111) electrode could impede the rate of Cu2+ reduction, thereby inhibiting rather than accelerating electrodeposition of copper under the present conditions. Real-time STM imaging revealed random nucleation of copper adatoms on Pt(111), followed by lateral growth of Cu nuclei upon further deposition. Segregated domains of (̄3 × ̄3)R30°, ascribable to MPS and chloride adspecies, were observed atop a monolayer of Cu deposit prior to the commencement of bulk Cu deposition. With a small overpotential (< 20 mV), multilayer copper was electroplated on Pt(111) in a layered manner, producing atomically smooth Cu deposit capped by patches of (3 × 3) MPS. By contrast, the Cu deposit on MPS-modified Pt(111) in 0.1 M HClO4 was decidedly rough, as reported earlier.