Surface-enhanced infrared absorption spectroscopy is used to examine the co-adsorption of a selection of polyethers with Cl- under conditions relevant to superconformal Cu electrodeposition in CuSO4-H2SO4 electrolytes. In 0.1 mol/L H2SO4, a potential-dependent mixed SO4 2--H3O+/H2O layer forms on weakly textured (111) Cu thin-film surfaces. With the addition of 1 mmol/L NaCl, the SO4 2--H3O+/H2O adlayer is displaced and rapidly replaced by an ordered halide layer that disrupts the adjacent solvent network, leading to an increase in non-hydrogen-bonded water that makes the interface more hydrophobic. The altered wetting behavior facilitates co-adsorption of polyethers, such as poly(ethylene glycols), polyoxamers, or polyoxamines. Interfacial water is displaced by co-adsorption of the hydrophobic polymer segments on the Cl--terminated surface, while the hydrophilic ether oxygens are available for hydrogen bond formation with the solvent. The combined polyether-Cl- layer serves as an effective suppressor of the Cu electrodeposition reaction by limiting access of Cuaq 2+ to the underlying metal surface. This insight differs from previous work which suggested that polymer adsorption is mediated by Cu+-ether binding.