NOVELTY - Electrolyte membrane comprises: (a) a host polymer comprising a fluoropolymer molecular chain consisting of a segment; and (b) a proton conductive polymer comprising a polymer chain being a co-polymerization product of a low molecular weight polymerizable proton conductor monomer consisting of an acidic group having a dissociable proton and at least one polymerizable functional group, with a crosslinking agent. The molecular chains of the host polymer and the proton conductive polymer form an interpenetrating polymer network.    USE - The electrolyte membrane is useful in a membrane electrode assembly of a fuel cell (all claimed), which is useful in vehicles and residential cogeneration systems.    ADVANTAGE - The electrolyte membrane can be simply prepared with less smaller number of steps and reduced cost, and can be operated under high temperatures (e.g. 100 degrees C or higher) and non-humidified or low humidity condition.    DETAILED DESCRIPTION - Electrolyte membrane comprises: (a) a host polymer comprising a fluoropolymer molecular chain consisting of a segment having a structure of formulae (-CF2-CF(M)CH2-CF2-), where the segment is defluorinated or dehydrofluorinated and chemically crosslinked by a low molecular weight basic compound having at least two amino groups; and (b) a proton conductive polymer comprising a polymer chain being a co-polymerization product of a low molecular weight polymerizable proton conductor monomer consisting of an acidic group having a dissociable proton and at least one polymerizable functional group, with a crosslinking agent. The molecular chains of the host polymer and the proton conductive polymer form an interpenetrating polymer network.    M = -CF3, -CF2H, or -CFH2.    INDEPENDENT CLAIMS are included for:    (1) a membrane electrode assembly comprising a cathode, an anode, and an electrolyte membrane located between the cathode and the anode, where the electrolyte membrane comprises the electrolyte membrane; and    (2) preparing the electrolyte membrane, comprising reacting, in an organic solvent, the fluoropolymer with the low molecular weight basic compound, preparing a mixed solution comprising the low molecular weight polymerizable proton conductor monomer and the crosslinking agent which crosslinks the low molecular weight polymerizable proton conductor monomer, removing the organic solvent from the mixed solution to provide a precipitate, performing a first crosslinking of the fluoropolymer with the low molecular weight basic compound contained in the precipitate to provide a crosslinked fluoropolymer, co-polymerizing the low molecular weight polymerizable proton conductor contained in the precipitate with the crosslinking agent to provide the polymerized product of the low molecular weight proton conductor, and performing a second crosslinking of the polymerized product of the low molecular weight proton conductor and the crosslinked fluoropolymer to provide an interpenetrating polymer network.