Proton-exchange membranes, for possible use in H-2/O-2 and direct methanol fuel cells have been fabricated from poly[bis(3-methylphenoxy)phosphazene] by first sulfonating the base polymer with SO3 and then solution-casting thin films. The ion-exchange capacity of the membrane was 1.4 mmol/g. Polymer crosslinking was carried out by dissolving benzophenone photoinitiator in the membrane casting solution and then exposing the resulting films after solvent evaporation to UV light. The crosslinked membranes look particularly promising for possible proton exchange membrane (PEM) fuel cell applications. A sulfonated and crosslinked polyphosphazene membrane swelled less than Nafion 117 in both water and methanol. Proton conductivities in crosslinked and non-crosslinked 200 mu m thick water-equilibrated polyphosphazene films at temperatures between 25 degrees C and 65 degrees C were essentially the same and only 30% lower than those for Nafion 117. Additionally, water and methanol diffusivities in the crosslinked polyphosphazene membrane were very low (less than or equal to 1.2 x 10(-7) cm(2)/s). Sulfonated/crosslinked polyphosphazene films showed no signs of mechanical failure (softening) up to 173 degrees C and a pressure of 800 kPa and did not degrade chemically when soaked in a hot hydrogen peroxide/ferrous ion solution.