Many polymer electrolytes used in practical applications contain a low molecular weight plasticizer. Plasticizers can be used to change the mechanical and electrical properties of polymer electrolytes by reducing the degree of crystallinity; and lowering the glass transition temperature T-g. In this study the ionic conductivity has been determined for a (PEO)(9)LiCF3SO3 polymer electrolyte with added plasticizers. The samples were in the form of thin films with the thickness 0.1-0.5 mm and ethylene carbonate (EC), propylene carbonate (PC) or a mixture of ethylene carbonate and propylene carbonate (EC:PC) were used as plasticizers.The ionic conductivity increases with increasing amount of plasticizer but the amount of plasticizer which can be added is limited since the films become too soft for use in practical applications. Without any plasticizer, the complex (PEO)(9)LiCF3SO3 only has a conductivity of 2.5 x 10(-5) S cm(-1) at 332 K. Adding 50% of the plasticizers by mel. weight of PEO to the (PEO)(9)LiCF3SO3 complex yielded mechanically stable films with an ionic conductivity of 9.0 x 10(-4) S cm(-1) with EC and 5.2 x 10(-5) S cm(-1) with PC at the same temperature.