Four phosphazene polymers : poly[(3-methylphenoxy)(phenoxy)phosphazene], poly[(4-methylphenoxy)(phenoxy)phosphazene], poly[(3-ethylphenoxy)(phenoxy)phosphazene] and poly[(4-ethylphenoxy)(phenoxy)phosphazene] were sulfonated in solution with SO3 and cast into membranes from N,N-dimethylacetamide or 1-methyl-2-pyrrolidinone solvents at a temperature of 80 degrees C. Methylphenoxy polymers were resistant to degradation and the sulfonation degree was easily controlled. The ethylphenoxy polymers underwent severe degradation during sulfonation and were unusable as membranes. Depending on the molar ratio of SO3 to the polymer mer, water insoluble membranes from the poly[(methylphenoxy)(phenoxy)phosphazenes] had an ion-exchange capacity ranging from near 0 to 2.3 mmol/g, an ac impedance in 0.1 N NaCl between 48 kohm m and 0.04 ohm m, and swelling in water (SO3H-form) from 0.1 to 0.9 g/g. Poly[(3-methylphenoxy) (phenoxy)phosphazene] was found to be the best starting material, in terms of the ease in controlling the degree of sulfonation and the highest polymer ion-exchange capacity for a water insoluble membrane.