Polymer electrolytes of poly(ethylene oxide) matrix with lithium imide salt LiN(CF3SO2)(2) were prepared by casting from solution. Thin films with compositions corresponding to molar ratios 6:1, 3:1 and 2:1 EO:Li were investigated by impedance spectroscopy, impedance spectroscopy simultaneous with polarizing microscope observation, X-ray diffraction and differential scanning calorimetry. The presence of PEO:LiTFSI stoichiometric complexes was found to significantly decrease conductivity at temperature of crystallization, which indicates that those complexes should be regarded as poorly conductive. Changes of properties of amorphous phase related to crystallization were also observed. Crystallization induced phase segregation, which in some cases caused considerable shift of the glass transition temperature of amorphous phase remaining in a semicrystalline system. For PEO:LiTFSI electrolyte with molar ratio of 3:1 EO:Li this effect was found to be responsible for enhancement of conductivity of semicrystalline sample in respect to the amorphous one, which was observed at low temperatures. Phase separation involving precipitation of LiTFSI salt was also found to be a likely explanation for significant enhancement of conductivity for PEO:LiTFSl 2:1 electrolyte subjected to rapid cooling below the glass transition temperature. (c) 2007 Elsevier Ltd. All rights reserved.