In this work, an industrially produced polymer electrolyte based on the poly(ethylene oxide) PEO-LiTFSI is studied. The evolution of the impedance spectra of symmetric cells Li/PEO-LiTFSI/Li with the aging time at 90degreesC is presented. The variation of impedance spectra as a function of temperature and electrolyte geometry is also shown, especially the low frequency part (up to 0.5 mHz). An equivalent electrical circuit is proposed to describe the whole spectra. The major contributions to the impedance of this interface are identified. From the low frequency contribution, the salt diffusion coefficient is determined. Finally, we use a simple model of the surface layer to gain some insight into its properties. (C) 2003 The Electrochemical Society.