Polymer electrolyte films prepared from poly(ethylene oxide) and lithium hexafluoroarsenate with varying composition of dibutylphthalate, the plasticizing agent, are studied by X-ray diffraction and AC impedance. A film containing 0.09 mol of dibutylphthalate is an optimum electrolyte composition by virtue of its free-standing ability with reasonably high conductivity. A film with this composition exhibits an enhanced amorphous character and a reduced energy barrier to segmental motion which favours lithium-ion conduction through the film. The temperature dependence of the complex film, with and without plasticizer, appears to obey the Arrhenius Law. The activation energy is 0.578 kJ/mol, which is considerably lower than the value of 10 kJ/mol that is obtained for a film without a plasticizer.