A hydrophilic cross-linked polymer obtainable by copolymerization of hydrophobic and hydrophilic monomers that give a cross-linked hydrophilic polymer on polymerization; a monomer including a strongly ionic group; and water is useful as the membrane in an assembly that can be used in an electrolyter or fuel cell.More generally, a membrane electrode assembly comprises electrodes and an ion-exchange membrane which comprises a hydrophilic polymer including a strongly ionic group.A method for producing a membrane electrode assembly comprising electrodes and an ion-exchange membrane, comprises introducing between the electrodes a material or materials from which the membrane can be formed, and forming the membrane in situ.