The physical and gas-transport properties of Fe3+ cross-linked Nafion membranes were examined. Wide-angle X-ray diffraction results revealed a lower crystallinity for Nafion Fe3+ but showed essentially no changes in the average chain spacing upon cation exchange of Nafion H+. Raman and Fourier transform infrared spectroscopy techniques qualitatively measured the strength of the ionic bond between the Fe3+ cations and sulfonate anions. Thermal gravimetric analysis indicated that the incorporation of Fe3+ adversely affected the thermal stability of Nafion due to the catalytic decomposition of perfluoroalkylether side chains. Gas sorption isotherms of Nafion Fe3+ measured at 35 °C up to 20 atm exhibited a linear sorption uptake for O2, N2, and CH4 following Henry's law and slight concave behavior for CO2. Pure-gas permeation results showed reduced gas permeability but higher permselectivities compared to Nafion H+ with αN2/CH4 = 4.0, αCO2/CH4 = 35, and αHe/CH4 = 733 attributable to the strong physical cross-linking effect of Fe3+ that caused chain stiffening with enhanced size-sieving behavior. Gas mixture permeation experiments using 1:1 molar CO2/CH4 feed demonstrated reduced CO2 plasticization for Nafion Fe3+. At 10 atm CO2 partial pressure, CO2/CH4 selectivity decreased to 28 from the pure-gas value of 35, which was a significant improvement compared to the performance of a Nafion H+ membrane.