Sulfinated PEEK was synthesized via reduction of sulfochlorinated PEEK. The degree of sulfination was determined by redox titration. By partial reduction of sulfochlorinated PEEK, PEEK carrying both SO2Cl and sulfinate groups could be obtained. From these polymers, covalently cross-linked ionomer membranes were prepared by reaction with the cross-linker diiodobutane and subsequent hydrolysis of the sulfochloride groups by aqueous post-treatment. The membranes show strongly reduced swelling connected with good H+-conductivity. Moreover, due to the fact that in these membranes all macromolecules are integrated into the covalent network, no leaching out of soluble substance took place during immersion in 90°C hot DMAc for 5 days. One of the membranes was tested in a DMFC and exhibited stable performance up to T =130°C.