Abstract The successful formation of radiation-induced crosslinking in Nafion ® N117CS membranes was clearly demonstrated by tensile testing, a methanol uptake measurement, thermo-mechanical analysis (TMA), and fluorine-19 MAS NMR ( 19F NMR) spectroscopy. The possibility of radiation-induced crosslinking of Nafion ® had been hardly considered with regard to the thermal stability of Nafion ® membranes, since the scission of perfluoroalkylether pendant-chains in Nafion ® membranes in their protonated form begins at 523 K. To improve the thermal stability of Nafion ® membranes, Nafion ® membranes were soaked in sodium chloride prior to irradiation. The sodium-exchange Nafion ® membranes were irradiated with gamma rays in an argon atmosphere at temperatures ranging from room temperature to 618 K. The irradiated membranes were re-exchanged to the protonated form. The 19F NMR results for the alcohol-swollen membranes indicate peaks which originated due to radiation-induced crosslinking. An increase in percent elongation at break, a decrease in methanol uptake, and a decrease in the softening temperature at a differential TMA peak for membranes irradiated at 598 K compared to received membranes also add support for our new finding.