Poly(ether ether ketone) (PEEK) containing sodium sulfonate groups can be prepared directly from sodium 2,5-dihydroxybenzenesulfonate and 4,4'-difluorobenzophenone. This method is more advantageous than sulfonation of PEEK, avoiding possible degradation and crosslinking. The polymer shows high permeability of water vapor and high selectivity of permeation of water vapor versus nitrogen and can be used as gas dehumidification membrane material.