Poly(ether ether ketone) with high content of sodium sulfonate groups prepared directly from 5,5＇-carbonylbis(2-fluorobenzene sulfonate), 4,4＇-difluorobenzophenone and phenolphthalein via aromatic nucleophilic substitution reaction. The polymer showed excellent thermal stability, high permeability of water vapour, high permselectivity of water vapour over nitrogen, and high permselectivity of anions.