The low cost proton exchange membrane was prepared by cross-linking water soluble sulfonated-sulfinated poly(oxa-p-phenylene-3,3-phthalido-p-phenylene-oxa-p-phenylene-oxy-phenylene) (SsPEEK-WC). The prepared cross-linked membrane became insoluble in water, and exhibited high proton conductivity, 2.9 x 10(-2) S/cm at room temperature. The proton conductivity was comparable with that of Nafion (R) 117 membrane (6.2 x 10(-2) S/cm). The methanol permeability of the cross-linked membrane was 1.6 x 10(-7) cm(2)/s, much lower than that of Nafion (R) 117 membrane. (c) 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.