A group of (PEO)LiClO4-Li1.3Al0.3Ti1.7(PO4)(3) [where PEO is poly(ethylene oxide)] composite polymer electrolyte (CPE) films was prepared by the solution-casting method. In each film, the ethylene oxide/lithium ratio of 8 and the Li1.3Al0.3Ti1.7(PO4)(3) concentration of 15 wt % were fixed, but the number-average molecular weight of PEO was varied (from 5-7 x 10(4) to 10(6), 2.2-2.7 x 10(6) 3-4 X 10(6), 4-5 x 10(6), and 5.5-6 x 10(6)). Several techniques, including X-ray diffraction, differential scanning calorimetry (DSC), scanning electron microscopy (SEM), and electrical impedance spectroscopy (EIS), were used to characterize the CPE films. LiClO4 had a strong tendency to complex with PEO, but Li1.3Al0.3Ti1.7(PO4)(3) was instead dispersed in the PEO matrix. DSC analysis revealed that the amorphous phase was dominant in the CPE films, although the PEOs before use were quite crystalline. An SEM study showed smooth and homogeneous morphologies for the films with low-molecular-weight PEO and dual-phase characteristics for those with high-molecular-weight -PEO. The EIS results indicated that the CPE films were all ionic conductors, and the conducting behavior obeyed the Vogel-Tamman-Fulcher (VTF) equation. The parameters in the VTF equation were obtained and discussed with respect to the PEO molecular weights and the crystallinities of the CPE films. Of all the films, the one with PEO with the smallest number-average molecular weight of 5-7 x 10(4) had the maximum conductivity, that is, 1.590 x 10(-5) S/cm at room temperature and 1.886 X 10(-3) S/cm at 373 K. (c) 2006 Wiley Periodicals, Inc.