SOLID POLYMER ELECTROLYTE COMPOSITION, ION-EXCHANGE MEMBRANE, MEMBRANE ELECTRODE ASSEMBLY, AND FUEL CELL

To provide a solid polymer electrolyte composition that can improve oxidation resistance without causing decrease in proton conductivity, elution of added components and the like, and an ion-exchange membrane, a membrane electrode assembly and a fuel cell, each using the composition. It is possible to obtain a solid polymer electrolyte composition having excellent oxidation resistance without causing decrease in proton conductivity, elution of added components and the like, by mixing an aromatic hydrocarbon-based polymer electrolyte, a phosphorus-containing polymer compound and a metallic element.