The microstructure of sulfonated poly [bis(3-methylphenoxy)phosphazene] was studied using wide- and small-angle X-ray diffraction. A reflection peak, attributed to the presence of ionic clusters, was observed in the small-angle X-ray diffraction patterns of hydrated and dry polymers with an ion-exchange capacity (IEC) greater than or equal to0.6 mmol/g. The Bragg spacing from the ionic cluster structure was about 30 Angstrom for the nonhydrated polymer and 50 to 90 Angstrom for fully hydrated films. The effects of IEC, cation form of the polymer, temperature, and polymer water content on the cluster structure were investigated. The specific proton conductivity of water-swollen, sulfonated poly-[bis(3-methylphenoxy)phosphazene] films at 25 degreesC increased with increasing IEC, with a maximum conductivity of 0.1 S/cm at a polymer ion-exchange capacity of 1.6 mmol/g. The water-content percolation threshold for conductivity was between 17.5 and 25 vol %, and decreased with polymer IEC. The temperature dependence of proton conductivity for 1.2 mmol/g IEC poly[bis(3-methylphenoxy)phosphazene] membranes exhibited Arrhenius behavior with an apparent activation energy of 27.8 and 36.7 kJ/mol for crosslinked and noncrosslinked polymers, respectively.