In this article, a study of the aging of conducting Polyaniline-Polystyrene blends using X-ray photoelectron spectroscopy (XPS) and UV-visible-near IR analysis is presented. The physicochemical results are compared to those obtained by electrical measurements. XPS results confirm the existence of an oxidation process also deduced by the electrical conductivity studies. The N1s and S2p core level spectra decomposition allows to show that a deprotonation process and cyclization of tertiary amine occur during aging. The absorption spectrum shows a decrease of delocalized charges and the apparition of localized polarons after a long aging time. All these mechanisms are responsible of the electrical conductivity decrease observed during aging at elevated temperature. The results are presented for films of PANI-CSA-PSt blends, but the conclusions can be extended to pure conducting PANI-CSA films. (C) 2003 Wiley Periodicals, Inc.