Water sorption and mechanical properties of perfluoro sulfonated acids (PFSAs), Aquivion and Nafion, are compared under environmentally controlled conditions from 25 to 120 degrees C and water activities of 00.95. Under dry conditions Nafion and Aquivion have thermal transitions at 60 degrees C and 95 degrees C, respectively, where the elastic modulus decreases rapidly with increasing temperature. Below their thermal transition temperatures water sorption plasticizes both PFSAs; the elastic moduli decrease with increasing water activity. Above the thermal transition water sorption stiffens both polymers; increasing the water activity from 0 to 0.01 increases the elastic moduli by a factor >10. Plasticization and stiffening are reversible with changing water activity at constant temperature. The thermal transition in PFSAs is suggested to result from reversible clustering of ionic groups. The higher thermal transition temperature for Aquivion is suggested to reduce the risk of membrane thinning and failure due to creep. (c) 2013 Wiley Periodicals, Inc. J. Polym. Sci. Part B: Polym. Phys. 2013, 51, 915925