Polyphosphazenes with two different types of side group structures based on aryloxy and oligo-ethyleneoxy units have been synthesized and examined as solid polymer electrolytes. The aryloxy groups improve the mechanical properties, while the oligoethyleneoxy units facilitate lithium ion transport. Two different polymer structures have been studied: co-substituent phenoxy/oligo-ethyleneoxy polyphosphazenes with the general structure [NP(OC6H5)(OCH2CH2),OCH3)(2)] and singlesubstituent phosphazenes with aryloxy groups that bear oligoethyleneoxy units in the para position of the aromatic rings, having the formula {NP[OC6H4(OCH2CH2)(x)OCH3](2)}. Polymers of both types contain the same 50:50 molar ratio of aryloxy/ oligoethyleneoxy groups, but their properties vary significantly. In general, the gum-like co-substituent polymers have lower glass transition temperatures and higher ionic conductivities. The single-substituent polymers are tough materials that form excellent films, but have slightly lower ionic conductivities. Factors that affect the ionic conductivity, including glass transition temperature, etheric oxygen concentration, and steric shielding of the phosphazene backbone are discussed. (C) 2003 Elsevier Science B.V. All rights reserved.