This research is focused in finding new methods for the modification of some polymers used in the fabrication of membranes for fuel cell applications. The modification in the physical or chemical properties of these polymers is necessary to improve the performance of these membranes. The polymeric membranes considered in this study are: NafionŽ (a perfluorosulfonic acid membrane), several sulfonated triblock copolymers such as SEBS (styrene-ethylene-isobutylene-styrene), SIBS (styrene-isobutylene-styrene); polybenzimidazole (PBI); and blends of these membranes with polyvinyl alcohol. In the present study, we have measured the transport properties (permeability) of NafionŽ membranes processed with SC CO2 and ten different cosolvents. The co-solvents studied included: acetic acid, acetone, acetonitrile, cyclohexanone, ethanol, isopropanol, methanol, methylene chloride, and tetrahydrofuran. Transport properties were measured using a FT-IR analytical technique. The results obtained show a reduction in the permeability of methanol through NafionŽ. The magnitude orders for permeability of the processed NafionŽ membranes was 10-6-10-9 cm2/s, while SIBS membranes showed a order of 10-7 cm2/s in methanol permeability. This can be attributed to a rearrangement of the CO2-phillic perfluorinated groups blocking the passage of methanol through the membrane. Additional experiments to corroborate the improvements in the fuel cell membrane include: proton conductivity, ion exchange capacity, and the actual fuel cell run. The next step for this investigation includes the use of polymer blends of these polymeric materials with the goal of tuning the permeability and proton conductivity of the membranes. These membranes can also be used in new materials for applications in chemical protective clothing, and even as chemical sensors.