The adsorption of polyethylene glycol and chloride ion (PEG-Cl-) on (i) air-oxidized Ru (native oxide), (ii) "activated" Ru (electrolytic reduction of the native oxide), and (iii) underpotential deposited (UPD) Cu on activated Ru was examined in situ using spectroscopic ellipsometry. In the absence of Cl-, PEG adsorption was minimal at all relevant potentials on the activated Ru and Cu UPD surfaces characterized in this study. On activated Ru, the addition of Cl- ion resulted in enhanced PEG coadsorption. At potentials relevant to Cu UPD, a three-component PEG-Cl--Cu layer formed independent of the order of additive addition to the electrolyte. The PEG-Cl--Cu UPD overlayer provided inhibition of subsequent Cu overpotential deposition. At potentials positive of Cu UPD, a monolayer oxide film formed on Ru that inhibited PEG adsorption even in the presence of Cl-. Slight PEG adsorption was observed on Ru native oxide surfaces, although there was no enhancement observed in the presence of Cl-. Ru oxidation exerted a strong effect on the adsorption of additives that was directly relevant to the nucleation and growth of electrodeposited Cu.