A method for the synthesis of phenyl phosphonic acid functionalized poly [aryloxyphosphazene] membranes is described. Membranes of polymers with total ion-exchange capacities between 1.17 and 1.43 meq. g(-1) were cast from N,N-dimethyl-formamide and found to have proton conductivities between 10(-2) and 10(-1) S cm(-1). Equilibrium water swelling values were determined to be between 11 and 32% depending on the polymer structure. Methanol diffusion coefficients for both radiation cross-linked and non-cross-linked membranes in 3 M aqueous methanol at 80 degreesC and 2.8 bar were found to be at least 12 times lower than for Nafion 117, and 6 times lower than for a radiation cross-linked sulfonated polyphosphazene membrane.