A new method for the preparation of proton exchange membranes (PEMs), based on cross-linked sulfonated poly(ether ether ketones) is reported. The cross-linking involves thermally activated bridging of the polymer chains with polyatomic alcohols, which makes the polymer mechanically stronger and reduces its swelling by water. Although the cross-linking occurs through the sulfonic acid group, a major portion of acid functions is not involved in cross-linking and remains available for proton transfer. Owing to this, membrane conductivities are only somewhat reduced, leaving some with values above 2 x 10(-2) S/cm at room temperature. (C) 2004 Elsevier B.V. All rights reserved.