Au(111) electrodes have been modified with self-assembled monolayers (SAM) of 3-mercapto-1-propanesulfonic acid (MPS) and used as a substrate for Cu electrodeposition. Aqueous plating solutions contained 0.1 M H 2SO 4, low Cu concentrations (≤80 μM), and, optionally, 1.4 mM Cl ions. The deposition process was characterized by cyclic voltammetry (CV) and in-situ scanning tunneling microscopy (STM) as a function of the electrode potential. At potentials positive of Cu growth (≥0.7 V Rhe), freshly modified electrodes are covered by an ordered (5√ 3 × √21) MPS adlayer (α) both in Cl-free and Cl-containing electrolytes. The α adlayer becomes disordered at more negative potentials prior to the onset of Cu deposition (≤0.65 V RHe). In the potential regime of Cu underpotential deposition (UPD) (≈0.2-0.65 V RHe), the surface morphology strongly depends on the presence of Cl. In the absence of Cl, a transient, ordered Cu/MPS adlayer phase (δ) forms via 2D growth and covers the entire Au(111) surface. Subsequently, the δ phase transforms into a disordered Cu/MPS phase (σC u) with small, embedded Cu islands. In Cl-containing electrolyte, a disordered Cu/MPS/Cl phase (γ) nucleates at Au step edges or surface defects and spreads laterally. Cu islands form simultaneously within the γ phase. Two-dimensional growth of these islands results in a pure Cu-UPD layer. Overpotential deposition (OPD) proceeds via layer-by-layer mode with second layer nucleations at surprisingly small critical coverages (θC ≤≤0.5). Our observations differ significantly from those in previous studies, demonstrating that the Cu growth behavior critically depends on the concentrations of MPS, Cu, and Cl at the interface.