Potential-dependent adsorption of bis-͑3-sulfopropyl͒ disulfide ͑SPS͒, 3-mercaptopropyl sulfonate ͑MPS͒, chloride ͑Cl − ͒, and polyethylene glycol ͑PEG͒ on evaporated Cu thin films was examined using in situ spectroscopic ellipsometry at −0.65 V MSE , −0.75 V MSE , and −0.85 V MSE under quiescent conditions. The adsorption for the thiol compound was similar at all potentials examined, but the adsorption of the disulfide increased at more negative potentials. The influence of Cl − on the substrate was greatly diminished and PEG concurrently less strongly adsorbed at more negative potentials, as measured by ellipsometric responses. Both adsorbed MPS and SPS inhibited the specific adsorption of PEG in the presence of Cl − . Likewise MPS and SPS added to the electrolyte displaced preadsorbed PEG/ Cl − layers, with the displacement by the disulfide compound strongly potential dependent. At all potentials studied, the disulfide was less effective than MPS at displacement of preadsorbed PEG/ Cl − .