LiCoO2 thin-film electrodes were prepared by rf-sputtering and sol–gel methods. The prepared thin films contained different amounts of Co3O4 as an impurity depending on the preparation conditions. In situ FT-IR spectroscopic analyses on electrochemical oxidation of ethylene carbonate (EC) and diethyl carbonate (DEC) containing LiClO4 on the LiCoO2 thin-film electrodes were carried out. The electrochemical oxidations of EC and DEC occurred even at 3.8V versus Li/Li+ on the fresh LiCoO2. The solvents EC and DEC were oxidized to some compounds with carboxylic groups. In situ FT-IR spectra obtained for the LiCoO2 thin films prepared by different methods were compared each other.