State of the art manufacturing of semiconductor devices involves electrodeposition of copper for device wiring and more recently for through-silicon-vias (TSVs). The process depends on the use of electrolyte additives that affect the local deposition rate thereby resulting in superconformal, or bottom–up “superfilling” of trenches and vias. Quantitative descriptions based on mass balance analysis of the additives and their effect on the local deposition rate have been shown to be effective for describing Damascene feature filling (CEAC, Curvature Enhanced Coverage Mechanism) and larger scale TSV filling (S_NDR, S-shaped negative differential resistance models). Despite the success of these processes much remains to be known concerning the detailed molecular nature of the competitive co-adsorption processes involved in both the suppression and acceleration of the metal deposition reaction. Accordingly, the application of in situ surface science tools such as in situ STM and SEIRAS towards a better understanding of the additive interaction will also be detailed. These studies show that the addition of Cl^- to an acidified CuSO₄ electrolyte displaces co-adsorbed SO₄^2--water species from the Cu surface. The rapidly formed ordered halide overlayer disrupts the adjacent solvent hydrogen bond network to create a hydrophobic interface that facilitates co-adsorption of polyethers, such as poly(ethylene glycol) (PEG), polyoxamers (EPE), polyoxamines (TET), that are known to block the Cu deposition reaction by limiting access of Cu²⁺_aq to the underlying metal surface. When challenged by bis-(3-sodium sulfopropyl disulfide) (SPS), or mercaptopropane sulfonate (MPS), competitive disulfide/thiol adsorption leads to disruption and displacement of the polymer suppressor. The disulfide/thiol head group tethers SPS/MPS to the surface, via Cu adatoms or defects in the Cl^- covered surface, while the SO₃^- terminal group makes the interface hydrophilic which accounts for its anti-suppressor action. The potential dependence of competitive SPS adsorption and its dependence on the strength of polyether adsorption were measured.