The effect of thermal and chemical treatments on the structural change of poly(3,4-ethylenedioxythiophene)/poly(4-styrenesulfonate) (PEDOT:PSS) have been investigated using X-ray photoelectron spectroscopy (XPS), optical spectroscopy and conductivity measurements. Surface analysis and optical measurements of the samples showed that the thermal treatments have little effects on the structure of the film. A hydrochloric acid (HCl) etching modified the composition of the dopant, in particular, by dissolving the sodium salt in PSS. The resistivity of thermally treated polymer films is slightly increased as compared to pristine films while acid treated films shows a drastic decrease in resistivity, probably by a protonic doping of the polymer. In ITO/PEDOT:PSS systems, it has been proved that indium from the oxide has diffused through the polymer film, probably due to the decomposition of the ITO film in contact with the polymer suspension, which contains water. A thermal treatment of the system increased the diffusion process. With acid etching, indium impurities were removed, and the process may be used to improve the stability of the light emitting diodes. (C) 2003 Elsevier B.V. All rights reserved.