A new U-V-curable vinylic ether-containing cosubstituted cyclotriphosphazene, N3P3(O(CH2CH2O)(2)CH3)(3)(OCH2CH2OCH=CH2)(3), [V3M3], was synthesized with high yield. This cosubstituted cyclotriphosphazene was characterized by FT-IR, ESI-MS, and H-1,C-13, and P-31 NMR spectroscopy. A series of free-standing films with thickness of about 200 mu M of V3M3-based composite polymer electrolytes were then prepared by mixing V3M3 with various amounts of LiClO4 salt and alpha-Al2O3 and cured by UV irradiation. The cation-oxygen interactions in the cured polymer electrolytes were investigated by FTIR spectroscopy. The ionic conductivities of the cured polymeric membranes measured by ac impedance measurements showed that the best conductivity of the as-prepared solid polymer electrolyte film at room temperature was 3.5 x 10(-5) S/cm. The result of the cyclic voltammetry measurements revealed that the V3M3-based composite polymer electrolyte films were electrochemically stable up to about 4.2 V.