The adsorption of the additive polyethyleneglycol 8000 (PEG8) and its coadsorption with Cl- ions was investigated by cyclic voltammetry and linear potential scans in conjunction with simultaneous measurements of the frequency change of an electrochemical quartz crystal microbalance (EQCM). Data obtained from the studies of EQCM for solutions of HClO4 containing PEG8, shows the formation of a peak (IcI) in the potential range from 0.2 to 0.4 V during the cathodic potential scan, which is due to the adsorption of PEG8 onto Pt. Analysis of simultaneously recorded massograms and voltammograms revealed that the adsorption of PEG8 occurs via a non-Faradaic process, and that no adsorption of PEG8 is observed at the open circuit potential. As the concentration of PEG8 in the solution was increased over the range I X 10(-6) M less than or equal to [PEG8] less than or equal to 8 x 10(-6) M, the degree of coating by PEG8 on the Pt surface increased to 0.21. The presence of Cl- ions in the solution inhibited PEG8 adsorption, and the degree of inhibition gradually increased with increasing Cl- concentration. (C) 2003 Elsevier Ltd. All rights reserved.