Nafion (R)-117 was exposed to supercritical fluid (SCF) CO2 and a cation solution using two different approaches: first was processed with SCF CO2, and then exchanged using six different cations: K+1, Ca+2, Ba+2, Cu+2, Fe+3, and Al+3. The second method performed the cation substitution first, followed by the SCF CO2 processing. The resulting composite membranes were characterized using several techniques: thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), Fourier transforms infrared spectroscopy (FT-IR), small angle X-ray scattering (SAXS), and X-ray diffraction (XRD). These techniques were used to identify the changes in the chemical and thermal properties of the membranes, as well as to evaluate changes in the resulting morphologies and crystallinities. Proton conductivity and methanol permeability were measured to understand how the different approaches promoted or inhibited the transport of certain substances through the membrane. Significant differences in their thermal, physical and transport properties were observed when Nafion (R) was processed with SCF CO2 and exchanged with cations. (c) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013