NOVELTY - An ion-exchange polymer membrane comprises a blend of homogeneously sulfonated poly(phenylene oxide) (PPO) having a number average molecular weight of 15,00-10,000,000 and an ion charge density of 1-3.9 meq/g; and a poly(vinylidene fluoride) (PVF2) having number average molecular weight of 10,000-10,000,000, in a PPO-PVF2 weight ratio of 1-49 and less than 19-31.    USE - Blend membranes of this invention are suitable for solid polymer electrolytes, especially for the polymer electrolyte membrane (PEM) fuel cell, electrolyte double-layer capacitor, and for the rechargeable zinc-halide cell.    ADVANTAGE - The invention has improved membrane characteristics. The blend membrane of this invention has higher ionic conductivity, thus fuel cells having better performance can be obtained. The sulfonated PPO and PVF2 blend membranes can conduct ions under an electrical driving force, when comprised of more than 62 wt.% of PVF2. The resulting blend membrane is crystalline and hydrophobic. The resulting blend membrane also provide good barrier to many aqueous solutes, because of their low affinity to water. Further, the resulting blend membrane has reduced swelling.    DETAILED DESCRIPTION - An ion-exchange polymer membrane comprises a blend of homogeneously sulfonated poly(phenylene oxide) (PPO) having a number average molecular weight of 15,000-10,000,000, preferably 30,000-10,000,000, and an ion charge density of 1-3.9 meq/g; and a poly(vinylidene fluoride) (PVF2) having number average molecular weight of 10,000-10,000,000, in a PPO-PVF2 weight ratio of 1-49 and less than 19-31. The homogeneously sulfonated poly(phenylene oxide) is of chemical structure (I).    R1 and R2 = H, SO3H, and SO3M;    M = alkali metal, alkaline earth metal (preferably lithium, sodium, or potassium) or a transition metal;    n = integer greater than 40.    INDEPENDENT CLAIMS are also included for:    (a) a solid polymer electrolyte membrane fuel cell;    (b) an electrolyte double-layer capacitor; and    (c) a rechargeable zinc-halide cell, all containing an ion-exchange polymer membrane of this invention.