X-ray photoelectron spectroscopy (XPS) was used to determine the nature and composition of electrode/electrolyte interfaces forming during the 55 degrees C cycling of Li-based cells in ethylene carbonate: dimethyl carbonate LiPF6 electrolyte using a heat-treated stainless steel substrate as the positive electrode. From a classical analysis of the XPS C 1s, O 1s, F 1s, P 2p, and Li 1s core peak spectra complemented by an unusual detailed interpretation of XPS valence spectra, we could follow, as a function of the cell cycling history, the evolution and nature of the species constituting the organic/inorganic layer as well as determine its approximate composition. We have shown that this surface layer mainly consists of PEO oligomers (-CH2-CH2-O-)(n), carbonates Li2CO3 and/or CH3OCO2Li, LiPF6 salt, and of degradation products of the salt such as LiF and phosphates. Moreover, we give evidence that this layer does not only grow but also becomes richer in CH3OCO2Li and LiF species upon cycling. (c) 2005 The Electrochemical Society. All rights reserved.