The membrane durability is a critical issue for proton exchange membrane fuel cell development. Nafion (R) is usually considered as a reference membrane due to its high chemical stability. However, a condensation mechanism of the sullonic groups to fOrM anhydrides was evidenced when Nafioe (R) membranes were stored at 80 degrees C under humid atmospheres. In the present work, Nafion (R) 112 membranes and chemically stabilized Nafion (R) I' NRE212C5 membranes commercialized to replace Nafion (R)) 112 were studied under hygrothermal aging performed at 80 degrees C for two relative humidity conditions: 80% and 95%. The impact of the aging conditions was investigated on membrane water and gas transport properties over the whole range of water activity. For both membrane types, a progressive decrease of the water uptakes was observed as a function of aging time. The hygrothermal aging process was faster for Nafioe 112 membranes than for the chemically stabilized Nalioe' membranes, It became slower as the relative humidity of the aging atmosphere decreased from 95% to 80%. A water transport mechanism based on sorption and permeation data was proposed for the aged membranes taking account of the water/polymer interactions and of the progressive crosslinking of the Nation" hydrophilic phase. The gas transport properties were studied for H-2, O-2 and CO2 at different water activities. The decrease of the gas permeability coefficients observed for the aged membranes in comparison with the neat membranes confirmed the presence of gas diffusion pathways within the Nafion (R) hydrophilic phase. The role of these diffusion paths was discussed as a function of the membrane hydration state, as a function of the aging conditions and of the polarity and size of the diffusing gas molecules., (C) 2013 Elsevier By. All rights reserved.