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Random copolymers of the polyphosphazene [NPR2]n have been synthesized via living ionic polymerization with mixed substituents at the phosphorus atoms (i.e., R = bis(2-methoxy-ethyl)amino and n-propylamino). The polymers melt at 190 °C and start to decompose above 300 °C. Thin polymer electrolyte membranes were prepared by solution casting with dissolved lithium triflate (LiSO3CF3) and with NaI. The transparent membranes showed favorable mechanical properties below 100 °C. Tg values ranged between −50 and −36 °C. Membranes with 10 wt % LiSO3CF3 (corresponding to the atomic ratio Li/(O + N) = 1/30) showed rather low conductivities between 3.2 × 10-7 S cm-1 at 30 °C and 1.9 × 10-5 S cm-1 at 100 °C as determined from impedance measurements. The dispersion of 4 wt % Al2O3 nanoparticles in the polyphosphazene membranes with 10 wt % LiSO3CF3, however, leads to an increase of the conductivities by 2 orders of magnitude, that is, 1.0 × 10-5 S cm-1 at 30 °C and 1.5 × 10-3 S cm-1 at 100 °C. The heterogeneously dope...