Electrochemical and mechanical properties of thin poly(3,4-ethylenedioxythiophene) (PEDOT) films deposited on gold and platinum have been investigated in aqueous sulfuric acid solutions. At sufficiently positive electrode potentials overoxidation of the polymer takes place on both metals. Structural changes of the polymer layer caused by overoxidation (degradation) could be detected by using the electrochemical bending beam method. The impedance spectra recorded after overoxidation differed considerably from the impedance spectra of freshly made electrodes. The most interesting feature was the appearance of an arc (or a "depressed semicircle") at high frequencies in the complex plane impedance plot, which indicates that the charge transfer process at the metal/film interface is hindered by the degradation process. Changes of the effective mass of the film were studied by electrochemical quartz crystal microbalance (EQCM). The morphological changes of the PEDOT film during overoxidation were analyzed by scanning electron microscopy (SEM). (C) 2012 Elsevier Ltd. All rights reserved.