NOVELTY - An electrode electrolyte comprises branched polyarylene copolymer having structural unit (5) containing sulfonic acid group and structural unit (1) which does not contain sulfonic acid group.    USE - Electrolyte used in paste for electrode of membrane electrode assembly and polymer electrolyte fuel cells (all claimed) used as power source of motor vehicles and portable devices.    ADVANTAGE - The electrolyte provides membrane with high proton conductivity under high temperature, dimensional stability, hot water resistance and mechanical characteristics. The sulfonic acid group is introduced into the polymer without requiring a complicated sulfonation process. The polyarylene polymer is manufactured economically. The catalyst metal used in the manufacture of the polyarylene polymer can be recovered easily.    DETAILED DESCRIPTION - An electrode electrolyte comprises branched polyarylene copolymer having structural unit of formula (5) containing sulfonic acid group and structural unit of formula (1) which does not contain sulfonic acid group.    R01=1-40 structural units of formulae (1a) and -Q-B'- (1b);    A,D'=direct bond, -O-, -S-, -CO-, -SO2-, -SO-, -CONH-, -COO-, -(CF2)i-, -(CH2)j'-, -CR"2-, cyclo hexylidene or fluorenylidene;    R"=aliphatic hydrocarbyl, aromatic hydrocarbyl or halogenated hydrocarbyl;    R1-R39=H, fluoro, alkyl, alkyl halide optionally substituted by halo, allyl, aryl, nitro or cyano;    A=-CO- or -SO2-;    q,r,s=0-4;    B'=oxygen or sulfur;    Q=structure chosen from formulae (2-4);    H01=group of formula (2a, 2b);    W'=-CO-, -SO2-, -SO-, -CONH-, -COO-, -(CF2)i- or -C(CF3)2-;    Z=direct bond, -(CH2)i-, -C(CH3)2-, -O-, -S-;    V'=-CO-, -SO2-, -SO-, CONH-, -COO-, -C(CF2)e- or C(CF3)2-;    Z01=direct bond, -(CH2)f-, C(CH3)2-, -O-, -S-;    i,j',e,f=1-10;    Y'=-CO-, -SO2- or -SO-;    Ar=aromatic group having substituent chosen from -SO3H, -O(CH2)pSO3H or -O(CF2)pSO3H;    p=1-12;    t,u,j=0-2;and    l,k=1-4.    The structural units (1a, 1b) may couple together regularly or irregularly, and may form structure in which block consists only of unit of formula (1a) mutually coupled, and structure in which block consists only of unit -Q-B'- mutually coupled. An INDEPENDENT CLAIM is included for electrode paste, which contains the electrode electrolyte, catalyst particles and solvent.