The ionic conductivity of a solid polymer electrolyte based on a lithium salt dissolved in a poly(ethylene oxide) (PEO) host was significantly improved with the use of plasticizing agents. Either a plasticizing salt, solvent, or combinations of both were incorporated into free-standing solution-cast films of (PEO)(x)(LiCF3SO3). The greatest enhancement of conductivity was observed when a plasticizing salt (LiN[CF3SO2](2)) and solvent (diethyl phthalate) were combined, with the most notable improvement occurring below the melting temperature of PEO (approximate to 66 degrees C). Conductivity increased from 7.7 X 10(-7) S cm(-1) at 20 degrees C to 4.6 x 10(-5) S cm(-1) with the addition of both plasticizers.