Spectroscopic ellipsometry was used to examine the adsorption of poly(ethylene glycol) (PEG) and Cl- on polycrystalline Cu, Ag, and Au electrodes in sulfuric acid. In halide-free sulfuric acid, PEG adsorption on Cu and Ag is minimal at potentials positive of the estimated potential of zero charge (pzc). PEG adsorption on Au was also negligible in halide-free environments positive of the pzc, but slight adsorption was evident near the pzc. In contrast, in the presence of adsorbed halide, PEG coadsorption is clearly evident at potentials near or positive of the pzc for all metal samples studied. The PEG/Cl-coadsorbed film does not provide a significant barrier to outer-sphere electron-transfer reactions [i.e., Ru3+/(2+) (NH3)(6)], although inner-sphere reactions associated with electrodeposition of copper are effectively inhibited. The implications of these observations with respect to the role of Cu+ ions in the PEG-induced inhibition of Cu electrodeposition is discussed. © 2005 The Electrochemical Society. All rights reserved.