Electrochemical doping and dedoping processes in poly(3,4-ethylenedioxythiophene)-PEDOT have been studied by in situ ESR spectroelectrochemistry with an aim to investigate the behaviour of charge carrying species, generated and annihilated in these processes, with changing potential. As an extension of earlier studies, we investigated the behaviour of PEDOT at high oxidation potentials, beyond the limit of electrochemical stability of the polymer located at ca. 1.6 V. Past this limit, a sudden and irreversible drop in the concentration of spins together with narrowing of the ESR signal is observed. Changes in the spectroscopic response of the film are irreversible as evidenced by the course of the subsequent reduction process, which by no means resembles the reduction process of the polymer recorded within the potential window in which the polymer is stable. Supplemented by results of electrochemical studies, it is concluded that direct overoxidation of the polymer chain most probably leads to a decrease in the conjugation length of the polymer's delocalised pi-bond, through cross-linking of the polymer chains. Consequently, the remaining spins become trapped in isolated packets where they behave more like oligomer type radicals. (C) 2004 Elsevier Ltd. All rights reserved.