Nafion (R), a perfluorosulfonated ionomer used as a proton conducting membrane in fuel cells, is usually considered as highly chemically stable. However, the extruded Nafion (R) membrane was shown to be sensitive to hygrothermal aging when stored for very long periods at 80 degrees C under humid atmospheres. The aging process was interpreted as a condensation of the sulfonic groups to form anhydrides. The hygrothermal stability of solution-cast and chemically stabilized Nafion (R) (NRE212CS) commercialized to replace Nafion (R) is investigated through the determination of water sorption, conductivity, ion-exchange capacity and mechanical properties. The evolution of the chemical and physical structure was analyzed by IR, NMR, SAXS and WAXS. The lower rate of degradation for solution cast membranes compared to extruded membranes is attributed to a lower concentration of divalent cation contaminants such as Cu2+. The hygrothermal aging process is faster at high water content but it does not occur in liquid water. It is shown to progress from the surface to the bulk without major modifications of the local microstructure and crystallinity. The results are analyzed in the framework of the polymer ribbon model and it is suggested that the aging mainly acts within the bundles while the interbundle areas appear as the main conductive pathways. (c) 2013 Elsevier B.V. All rights reserved.