On the basis of a brief view to the studies on the structural evolution of perfluorosulfonic-acid ionomer membranes, a phenomenological description for the structural evolution of Nafion membranes from the anhydrous state to the swollen state at the molecular level is given. According to the description, it can be concluded that the pore-filling reinforced Nafion membranes are more effective than that of the Nafion composite membranes incorporating inorganic fillers to improve the properties of Nafion. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.