Cyclic voltammetry, differential capacity, and chronocoulometry were employed in a quantitative study of polyethylene glycol (Mw 20000) (PEG(20000)) 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), PEG(20000) 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 PEG(20000) concentration and more positive potential. In addition, the surface tension decreased as the quantity of PEG(20000) adsorbed on the surface rose with the increasing PEG(20000) concentration, an effect characteristic of surface active agents. The maximum surface concentration was found to be 4.59 x 10(-10) moloxygen cm(-2) for the highest PEG(20000) concentration studied (100 mu M). The Gibbs energy of adsorption (Delta G(ads)) was determined using the Henry adsorption isotherm. On the basis of the Delta G(ads) values obtained, it was concluded that the PEG(20000) molecules were weakly chemisorbed on the polycrystalline Au surface. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3551501] All rights reserved.