Ionic conductivities of complexes of poly(2-(4-vinyl-phenoxy)penta{[(methoxyethoxy)ethoxy]ethoxy}cyclotriphosphazene]) ((poly(STEP)) with alkali thiocyanates (K +, Na + and Cs +) have been measured. The same glass transition temperature ( T g) rise was observed for K + and Na + salt complexes, which was higher than that of Cs + salt complexes. The poly(STEP) complexes with these salts exhibit good ionic conductivities, especially for poly(STEP)-Cs + salt complexes, approaching 10 −4S/cm at 30°C and 10 −3S/cm at 90°C. The fast ion transport could be explained by the formation of a conducting phase consisting of a number of oxyethylene side chains attached to a cyclotriphosphazene. From the correlation between polymer characteristics and conductivities, the conduction appears to occur at a rate governed by the segmental mobility of side chains, cation-oxygen atom binding energy, and the structural factor for complex formation.