NOVELTY - A membrane humidifier for a fuel cell, comprises: a first flow field plate adapted to facilitate flow of a first gas to it; a second flow field plate adapted to facilitate flow of a second gas to it; and a polymeric membrane disposed between the first and second flow fields. The membrane adapted to permit transfer of water. The polymeric membrane comprises a first polymer having a polymer segment comprising perfluorocyclobutyl group and pendant side chain.    USE - In fuel cell system (claimed).    ADVANTAGE - The present membrane humidifier exhibits significantly improved performance compared to prior humidifiers. The present membrane humidifiers fulfill fuel cell humidification requirements. The present membrane humidifiers are compact, exhibit low pressure drop, and have high performance characteristics, for the automotive fuel cell humidification application.    DETAILED DESCRIPTION - A membrane humidifier for a fuel cell, comprises: a first flow field plate adapted to facilitate flow of a first gas to it; a second flow field plate adapted to facilitate flow of a second gas to it; and a polymeric membrane disposed between the first and second flow fields. The membrane adapted to permit transfer of water. The polymeric membrane comprises a first polymer having a polymer segment of formula (I).    Z1=-SO2X, -PO3H2 or -COX;    E1=aromatic containing moiety (preferably 16 groups of formulae as given in the specification, e.g. group of formulae (i)-(v));    A1=absent, O or chain extender having a carbon backbone;    X=-OH, halo, ester or -NH-S(=O)2-R4;    P1 and P2=absent, -O-, -S-, -SO-, -SO2-, -CO-, -NH-, NR2- or -R3-;    R2=1-25C alkyl, 1-25C aryl or 1-25C arylene;    R3=1-25C alkylene, 1-25C perfluoroalkylene or 1-25C arylene;    R4=trifluoromethyl, 1-25C alkyl, 1-25C perfluoroalkylene, 1-25C aryl or another E1 group;    Q1=fluorinated cyclobutyl moiety.    An INDEPENDENT CLAIM is included for a fuel cell system comprising: a fuel cell stack having a cathode side and an anode side; and the membrane humidifier.