Nafion(R) membranes commonly used in direct methanol fuel cells (DMFC) are typically limited by high methanol permeability. These membranes have phase-segregated sulfonated ionic domains in a perfluorinated backbone, which make processing difficult and limited by phase equilibria considerations. This study used supercritical fluids (SCFs) as a processing alternative, since the gas-like mass transport properties of SCFs allow for better penetration into the membranes and the use of polar cosolvents could also influence their morphology, thus fine-tuning their physical and transport properties. The SCF processing was performed at 40 degrees C and 200 bars using pure CO2 and CO2 with several polar cosolvents of different size and chemical functionalities like: acetic acid, acetone, acetonitrile, cyclohexanone, dichloromethane, ethanol, isopropanol, methanol, and tetrahydrofuran. Methanol permeability measurements revealed that the SCF processed membranes reduced the permeation of methanol by several orders of magnitude, especially with the use of some small polar cosolvents. Proton conductivity measurements, using AC electrochemical impedance spectroscopy, were on the order of 0.03-0.09 S/cm, which indicates that processing with SCF CO2 plus some cosolvents maintained the high proton conductivity while reducing the methanol permeability. The results are explained using XRD and SAXS. XRD analysis of the SCF processed samples revealed an increasing pattern in the crystallinity, which influenced the transport properties of the membrane. SAXS measurements confirmed the morphological differences that led to the changes in transport properties of the SCF processed membranes. Finally, processing flow direction (parallel versus perpendicular flow) played a major role in the morphological changes of this anisotropic membrane. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012