Sulfonated polyetheretherketones are polymers of some interest for preparing membranes as alternative to the expensive perfluorinated membranes (e.g. Nafion) presently used for fuel cell applications. Sulfonated poly(oxa-p-phenylene-3,3-phtalido-p-phenylene-oxa-p-phenileneoxy-phenyle ne) (PEEK-WC) with various degrees of substitution (DS) was prepared and then characterized by FT-IR analyses. Sulfonated PEEK-WC dense membranes were tested for their permeability to various gases and vapors. At 60 degreesC, the permeability coefficient of methanol in a sulfonated PEEK-WC membrane with 70% DS was about two orders of magnitude lower in comparison with that in a Nafion 117 membrane. Conductivity measurements were carried out by the impedance technique as a function of relative humidity (RH), at 100degreesC, and as a function of temperature, in the range 50-115 degreesC, at 100% RH. The proton conductivity strongly increases with temperature and DS and reaches 2.5 x 10(-2) S cm(-1) at 115 degreesC and 100% RH for DS = 0.82. The hydration of the sulfonated polymer was also determined under the same conditions used for conductivity measurements. (C) 2003 Elsevier B.V. All rights reserved.