Electrochemical oxidation behavior of non-aqueous electrolytes on LiCoO2 thin film electrodes were investigated by in situ polarization modulation Fourier transform infrared (PM-FTIR) spectroscopy, atomic force microscopy and X-ray photoelectron spectroscopy (XPS). LiCoO2 thin film electrode on gold substrate was prepared by rf-sputtering method. In situ PM-FTIR spectra were obtained at various electrode potentials during cyclic voltammetry measurement between 3.5 V vs. Li/Li+ and 4.2 V vs. Li/Li+. During anodic polarization, oxidation of non-aqueous electrolyte was observed, and oxidized products remained on the electrode at the potential higher than 3.75 V vs. Li/Li+ as a surface film. During cathodic polarization, the stripping of the surface film was observed at the potential lower than 3.9 V vs. Li/Li+. Depth profile of XPS also showed that more organic surface film remained on charged LiCoO2 than that on discharged one. AFM images of charged and discharged electrodes showed that some decomposed products deposited on charged electrode and disappeared from the surface of discharged one. These results indicate that the surface film on LiCoO2 is not so stable. (c) 2007 Elsevier B.V. All rights reserved.