A series of sulfonated poly(ether ether ketone ketone)s derived from bisphenol S were prepared by nucleophilic polycondensation. They showed high thermal resistance and good solubility. Most of the polymers were easily cast into tough membranes. The swelling of the membranes (6.02-16.02%) was lower than that of Nafion membranes, and the ion-exchange capacity of the membranes (0.67-1.44) was higher than that of Nafion membranes. The proton conductivity of the membranes was 0.022-0.125 s/cm. They could be used as proton-exchange membranes in fuel cells. (C) 2004 Wiley Periodicals, Inc.