NOVELTY - Formation of an electrolyte membrane comprises e.g. forming a mixture comprising a polyhydroxy compound, an aromatic polyhalo compound and an alkali metal hydroxide; arranging the mixture on a porous substrate; converting the mixture to form a crosslinked proton conductor; and sulfonating the proton conductor.    USE - The electrolyte membrane is useful for the preparation of an article, preferably fuel cell, battery, catalyst carrier or catalyst (all claimed). The electrolyte membrane is also useful as a polymeric catalyst or a catalyst carrier in water purification systems.    ADVANTAGE - The electrolyte membrane exhibits: high oxidation resistance to oxidizing agents, such as hydrogen peroxide; chemical resistance; excellent dimensional stability; improved mechanical strength; thermal stability; and good proton conductivity. The electrolyte membrane can be prepared without any formation of cracks or other deformations.    DETAILED DESCRIPTION - Formation of an electrolyte membrane comprises either forming a mixture comprising a polyhydroxy compound, an aromatic polyhalo compound and an alkali metal hydroxide; arranging the mixture on a porous substrate; converting the mixture to form a crosslinked proton conductor; and sulfonating the proton conductor, or forming a first mixture containing the first solvent, an alkali metal hydroxide and tris(hydroxyphenyl)ethane compound of formula (I); heating the first mixture; adding a (3-(3-sodiooxysulfonylphenyl)sulfonylphenyl)sulfonyloxysodium of formula (II) to the first mixture; adding a second solvent to the first mixture to obtain a second mixture, where the second solvent is isopropanol or dimethylsulfoxide; arranging the second mixture on the porous substrate, where the porous substrate is an expanded polytetrafluoroethylene; reacting the second mixture with the porous substrate of integrated crosslinked proton conductor to form the electrolyte membrane; and sulfonating the corsslinked proton conductor with a sulfonating agent to form a sulfonated crosslinked proton conductor.    R16-R20 = H, halo, 1-10C-alkyl, halo substituted 1-10C-alkyl, 6-10C-aryl or halo substituted 6-10C-aryl;    n, o, p = 1-4;    X = halo, preferably F; and    q, r = 1-3.    INDEPENDENT CLAIMS are included for:    (1) a process comprising deprotonating a tris(hydroxyphenyl)ethane compound of formula (III) with a sulfonated difluorodiphenyl sulfone compound of formula (IV) to obtain crosslinked polyethersulfone compound of formula (V), in pores of the porous substrate, and sulfonating sodium sulfoanted, crosslinked polyethersulfone compound of formula (VI) in presence of sulfuric acid and chlorosulfuric acid to obtain sulfonated crosslinked polyether sulfone compound of formula (VII) in the pores of the porous substrate; and    (2) an article comprising the porous substrate and the sulfonated, crosslinked proton conductor arranged within the pores the porous substrate.