NOVELTY - Membrane/electrode assembly (10) comprises a pair of electrodes; and an ion exchange membrane disposed between the electrodes, where the ion exchange membrane contains a sulfonated aromatic polymer of (I), and the minimum value of the internal resistance of the membrane/electrode assembly at 80degreesC and 120degreesC is 100 mOmega.cm2 or less and 600 mOmega.cm2 or less, respectively.    USE - (I) is useful in the fuel cell (claimed).    ADVANTAGE - The minimum value of the internal resistance of the membrane/electrode assembly at 80degreesC and 120degreesC is 100 (preferably 90) mOmega.cm2 or less and 600 (preferably 550) mOmega.cm2 or less, respectively. The ion conductivity of the binder in water at 80degreesC is 0.1 S/cm or more (all claimed). The membrane/electrode assembly has excellent power characteristics in a state of high temperature (120degreesC or higher) and low humidity (50% or lower). The ion exchange membrane has electroconductivity and good mechanical strength.    DETAILED DESCRIPTION - Membrane/electrode assembly comprises a pair of electrodes; and an ion exchange membrane disposed between the electrodes, where the ion exchange membrane contains a sulfonated aromatic polymer of formula (I), and the minimum value of the internal resistance of the membrane/electrode assembly at 80degreesC and 120degreesC is 100 mOmega.cm2 or less and 600 mOmega.cm2 or less, respectively.    m, n = a positive integer, where n/n+m is 0.001-1;    Y1 = S, S(O), S(O)2, C(O) and/or P(O)(C6H5);    Z = single bond, C(CH3)2, C(CF3)2, C(CF3)(C6H5), C(O), S(O)2 and/or P(O)(C6H5);    A = sulfonate group or sulfonate-containing moiety of formula -B1-(X-B2)m1SO3M;    B1, B2 = linking group;    X = S;    M = cation; and    m1 = an integer of at least one.    An INDEPENDENT CLAIM is included for a fuel cell containing the membrane/electrode assembly.    DESCRIPTION OF DRAWING(S) - The figure shows the structure of the fuel cell.    Membrane/electrode assembly (10)    Gaskets (14)    Charge/discharge ports (15)    Charge/discharge ports (16)    Collectors. (17)