Poly (ethylene oxide)-(PEO)-based composite polymer electrolytes are of great interest for solid-state-electrochemical devices. Among these materials, (PEO)(9)LiCF3SO3 or (PEO)(9)LiTf has been widely studied as a potential candidate. There have been many studies aimed at improving the ambient temperature ionic conductivity in this material either by incorporating plasticizers such as ethylene carbonate (EC) and propylene carbonate (PC) or by incorporating micro-sized or nano-sized inorganic fillers such as Al2O3, SiO2 or TiO2. However, to our knowledge, no report can be found in the literature on the combined effect of EC and Al2O3 on (PEO)(9)LiTf. This paper describes the combined effect of incorporating the Al2O3 filler and the EC plasticizer on the ionic conductivity enhancement of the (PEO)(9)LiTf electrolyte. Maximum conductivity enhancement has been achieved by optimizing the effects of the plasticizer and the ceramic filler. Nano-sized alumina filler-added, plasticized polymer electrolyte films (400-600 gm), prepared by common solvent casting method has been characterized by ionic conductivity and differential calorimetric measurements. The incorporation of 50 wt.% EC and 15 wt.% Al2O3 to the (PEO)(9)LiTf electrolyte showed a significant conductivity enhancement with sigma(RT) (max)=1.5 x 10(-4) S cm(-1) while retaining the mechanical strength of electrolyte films. As expected, the conductivity is enhanced by the plasticizer by reducing the crystallinity and increasing the amorphous phase content of the polymer electrolytes. The ceramic filler (Al2O3) also contributes to conductivity enhancement by promoting the above structural changes in the polymer electrolyte. It is evident that, an additional mechanism, directly associated with filler particles, would also be responsible for the conductivity enhancement caused by the filler. One possible explanation for this could be by creating additional sites for migrating ionic species through transient bonding with O/OH groups in the filler surface as suggested by previous workers. The decrease of (T)g, values of plasticized and filler-added polymer electrolyte samples seen in the DSC thermograms points towards the improved segmental flexibility of polymer chains, leading to increased mobility of conducting ions. 2007 Elsevier B.V. All rights reserved.