Considerable research effort has been devoted to lowering the useful operating temperature of PEO-LiX polymer electrolytes to the ambient region. The most common approach has been that of adding liquid plasticizers, such as ethylene carbonate (EC) or propylene carbonate (PC), to the PEO-LiX matrix. In this paper, we have extensively studied EC plasticized LiCF3SO3.P(EO)(10) and LiClO4.P(EO)10, measuring the conductivity, thermal characteristics and X-ray diffraction patterns of the electrolyte films. A very significant feature of the results is that the enhanced conductivity of the as-prepared films is transient and is not maintained on thermal cycling under vacuum. The systems are not thermodynamically stable due to a loss of the bulk of the plasticizer and this will impair their applications in devices.