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 temperatureT_g. In this study the ionic conductivity has been determined for a (PEO)₉LiCF₃SO₃ polymer electrolyte with added plasticizers. The samples were in the form of thin films with the thickness 0.1–0.5mm 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)₉LiCF₃SO₃ only has a conductivity of 2.5×10⁻⁵Scm⁻¹ at 332K. Adding 50% of the plasticizers by mol. weight of PEO to the (PEO)₉LiCF₃SO₃ complex yielded mechanically stable films with an ionic conductivity of 9.0×10⁻⁴Scm⁻¹ with EC and 5.2×10⁻⁵Scm⁻¹ with PC at the same temperature.