Solid polymer electrolytes based on poly(trimethylene carbonate) and poly(ethylene oxide) interpenetrating networks doped with lithium hexafluoroantimonate have been prepared and characterized by measurements of ionic conductivity, cyclic voltammetry, differential scanning calorimetry and thermogravimetry. Electrolytes with lithium salt compositions of n between 5 and 15 (where n represents the total number of cation-coordinating units per lithium ion) and a host polymer component of -(O=COCH2CH2CH2O)n- units contributing between 5 and 15 wt% of the electrolyte formulation, were prepared by co-dissolution in acetonitrile. The solvent casting technique was used to produce flexible, transparent and free-standing films.