Solid polymer electrolytes consisting of poly(ethylene oxide) PEO and lithium trifluroacetate (CF3COOLi) with various salt mass fractions were prepared by the solvent casting method using acetonitrile. Temperature and concentration dependent impedance spectroscopy, as well as thermal analysis suggest the existence of a complex in the blends with an EO/Li ratio corresponding roughly to 4:1. The dc conductivity (σ0) of the blends were very sensitive to the temperature (T) and their salt mass fraction (x), showing values in the range of 10-5 to 10-2 (S cm–1) at 330 K as the salt concentration was increased. The enhancement of conductivity with increasing temperature (5 orders of magnitude when the temperature changes from 300 to 353 K) was attributed to the high mobility of the Li+ ions as a consequence of the chain polymer flexibility and the increase of the free volume for ionic migration. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)