Cyclic voltammetry, differential capacity, and chronocoulometry were employed in a quantitative study of polyethylene glycol (Mw 20000) (PEG₂₀₀₀₀) adsorption onto a polycrystalline Au substrate. The surface tension, film pressure, relative Gibbs surface excess, and the Gibbs energy of adsorption were determined as a function of the electrode potential. The results indicated that, in the potential interval studied (−0.9 to 0.35 V vs saturated calomel electrode), PEG₂₀₀₀₀ adsorbed onto the polycrystalline Au surface via its oxygen atoms as a flat submonolayer of rodlike molecules. The adsorbed mass increased considerably with the increasing PEG₂₀₀₀₀ concentration and more positive potential. In addition, the surface tension decreased as the quantity of PEG₂₀₀₀₀ adsorbed on the surface rose with the increasing PEG₂₀₀₀₀ concentration, an effect characteristic of surface active agents. The maximum surface concentration was found to be 4.59 × 10₋₁₀ mol_oxygen cm₋₂ for the highest PEG₂₀₀₀₀ concentration studied (100 μM). The Gibbs energy of adsorption (ΔG _ads) was determined using the Henry adsorption isotherm. On the basis of the ΔG _ads values obtained, it was concluded that the PEG₂₀₀₀₀ molecules were weakly chemisorbed on the polycrystalline Au surface.