A composition containing a proton-conductive copolymer, a polymer electrolyte membrane containing the composition; a method of producing the membrane; and a fuel cell employing the membrane. The composition includes: a proton-conductive copolymer comprising a first styrene repeating unit, a second styrene repeating unit, and a dimethylsiloxane repeating unit; and a cross-linked polymer obtained from a cross-linking reaction between a siloxane oligomer having an unsaturated bond and a cross-linking agent. The cross-linked polymer has the same properties as the dimethylsiloxane repeating unit of the proton-conductive copolymer.