The influence of polyethylene glycol (PEG) on the atomic-scale interface structure and Cu growth behavior of Cu(001) in chloride-containing electrolyte was investigated by in situ surface X-ray diffraction. Studies in Cu-free solution unambiguously verify the presence of the ordered c(2 x 2) chloride adlayer in the presence of the polymer as well as reversible Cl adsorption/desorption and an order-disorder transition in the adlayer. The partially ordered chloride adlayer is found over a wider potential range as compared to PEG-free electrolyte, indicating stabilization of this adsorbate phase. Crystal truncation and superstructure rod measurements reveal that the polymer interacts more strongly with the partially c(2 x 2) Cl covered surface than with the close-packed c(2 x 2) adlayer at saturation coverage. In Cu-containing electrolyte layer-by-layer growth was observed at potentials <=-0.35 V vs. Ag/AgCl. The deposition rate was significantly reduced as compared to PEG-free electrolyte, providing clear evidence of inhibition on the atomic-scale. At more positive potentials a crossover to 3D growth and increasing surface roughness is found. (C) 2013 The Electrochemical Society. All rights reserved.