Poly[bis(3-methylphenoxy)phosphazene] was sulfonated in a solution with SO3 and solution-cast into 100-200-mu m-thick membranes from N,N-dimethylacetamide. The degree of polymer sulfonation was easily controlled and water-insoluble membranes were fabricated with an ion-exchange capacity (IEC) as high as 2.1 mmol/g. For water-insoluble polymers, there was no evidence of polyphosphazene degradation during sulfonation. The glass transition temperature varied from -28 degrees C for the base polymer to -10 degrees C for a sulfonated polymer with an IEC of 2.1 mmol/g. The equilibrium water swelling of membranes at 25 degrees C increased from near zero for a 0.04-mmol/g IEC membrane to 900 % when the IEC was 2.1 mmol/g. When the IEC was < 1.0 mmol/g, SO3 attacked the methylphenoxy side chains at the para position, whereas sulfonation occurred at all available aromatic carbons for higher ion-exchange capacities. Differential scanning calorimetry, wide-angle X-ray diffraction, and polarized microscopy showed that the base polymer, poly[bis(3-methylphenoxy)phosphazene], was semicrystalline. For sulfonated polymers with a measurable IEC, the 3-dimensional crystal structure vanished but a 2-dimensional ordered phase was retained.