The redox process mechanism responsible for the colour change in poly(3,4-ethylene-dioxythiophene) (PEDOT) was investigated in order to understand the nature and the role of the chemical species involved. PEDOT layers were obtained by chemical polymerisation with iron (III) p-toluenesulphonate. The films were then cycled several times in LiClO4/propylene carbonate (PC) electrolyte. The chemical composition of 'as-prepared' and cycled films (in the reduced and oxidised state) was studied by means of EDX and XPS ex situ analysis. The results point out the fundamental role the electrolyte anion plays in the process and provide quantitative information about the extent of anion exchange. (c) 2007 Elsevier B.V. All rights reserved.