Polymer electrolytes composed of poly(ethylene oxide) and lithium imide salt LiN(CF3SO2)(2) were prepared by casting from solution. The electrolytes were characterized by impedance spectroscopy, impedance spectroscopy simultaneous with polarizing microscope observation, X-ray diffraction and DSC. It was found, that the properties of amorphous phase, which remained in the semicrystalline PEO:LiTFSI electrolytes, depend on thermal history of the sample. Phase segregation, occurring when the molar ratio of salt in crystalline lamellae is different from the average molar ratio of electrolyte, was indicated as the mechanism of this effect. Phase segregation affected also the mechanism of crystallization and morphology of the crystalline phase. All these factors were found to influence strongly the ionic conductivity of the electrolyte. Due to shift of the glass transition temperature, at low temperature the conductivity of certain electrolytes was higher when they were in semicrystalline state than the conductivity of electrolytes in amorphous state. For electrolytes with high concentration of salt, additional effects of decoupling of the transport of ions from the motions of polymer matrix were observed. (c) 2007 Elsevier B.V. All rights reserved.