There is described a fuel cell or electrolytic cell comprising an ion-conductive polymeric material which includes a first repeat unit of formula (I): —(O-Ph1-CO-Ph1-O-Ph1-CO-Ph1)-, and a second repeat unit of formula (II): —(O-Ph2-O-Ph3-CO-Ph4)-, or of formula (III): (O-Ph2-O-Ph3-SO2-Ph4)-; wherein Ph1, Ph2, Ph3 and Ph4 independently represent phenyl moieties and wherein said second repeat unit is provided with ion-exchange sites. The polymeric material may include a third repeat unit which is amorphous.