Polymer electrolytes based on poly(ethylene oxide), (PEO), can be prepared as either a crystalline or an amorphous phase. A previous study of the 6:1 PEO:LiSbF6 electrolyte by Gadjourova et al. [Nature, 412 (2001) 520] reported that the crystalline phase has a higher ionic conductivity than the amorphous phase. Because this behavior is opposite to what is observed in other polymer electrolytes, it is important to understand the phases and phase transitions of this system. To this end, a detailed temperature-dependent infrared spectroscopic study was performed on P(EO)6LiSbF6 crystalline material. The spectroscopic data are complemented by thermal analysis and X-ray diffraction measurements. These studies show that the published P(EO)6LiSbF6 crystal structure with 2000 molecular weight PEO is stable from −100 to 90°C. The onset of a thermal transition is observed at about 110°C, corresponding to a crystal structure change. The new crystalline phase melts at about 130°C. The crystal structure of P(EO)6LiSbF6 made from 100 000 molecular weight PEO is stable with no intermediate structure from −100°C to the melting transition at 130°C, where severe decomposition is also observed.