The incorporation of siloxane networks in sulfonated polystyrene membranes by the sol-gel process was used to explore the possibility of developing low cost membrane for direct methanol fuel cells. A precursor solution of a hydrophobic siloxane network was allowed to diffuse and react into sulfonated syndio-polystyrene ionomeric membranes. The organic-inorganic hybrid domains so produced were able to reduce considerably the swelling of the ionomeric polymer in water, thereby increasing the dimensional stability of membranes. The physical and chemical properties of the sulfonated and hybrids membranes were examined by thermogravimetric analysis (TGA), infrared spectroscopy (FTIR) and small angle X-ray scattering (SAXS) analysis. The water uptake and the ionic conductivity were also evaluated at temperatures up to 60 degrees C. It was found that both the unmodified sulfonated membranes and the corresponding hybrid exhibited a two-phase morphology, consisting of crystalline lamellar domains embedded in the amorphous ionomeric polymer, featuring segregated hydrophobic/hydrophilic domains with characteristic separation lengths ranging between 3.9 and 1.7 nm. The presence of inorganic domains not only increased the dimensional stability of the membrane, by reducing the water uptake, but also decreased the rate of methanol crossover. Furthermore it was found that the inorganic network stabilizes the membrane morphology, enhancing the retention of proton conductivity after aging in water. POLYM. ENG. SCI., 48:2389-2399, 2008. (C) 2008 Society of Plastics Engineers