FT-IR ATR spectroscopy has been undertaken to elucidate how the structure of Nafion (TM) membranes is affected by the cationic form (i.e., H+, D+, and Na+) and water content. Three distinct hydration levels were considered: (a) completely dry (lambda equivalent to H2O/ - SO3- congruent to 0); (b) minimal hydration (lambda congruent to 2): and (c) fully hydrated (lambda congruent to 22). The measured spectra reveal that the associated cation and degree of hydration have a significant effect on the vibrational modes of the perfluoroether side chains of the ionomer. Two peaks ascribed to the - SO3X terminal groups and the features attributed to the COC linkages were clearly identified. The latter were studied in detail, and are shown to be diagnostic of the conformation of the side chains. A comprehensive framework of interpretation of the experimental results is proposed, with emphasis on: (a) the role played by the interactions between the - SO3X terminal groups of the different side chains; and (b) how the latter interactions are affected by the amount of H2O or D2O in the samples. In particular, reference is made to the formation and spatial distribution of the positively-charged species characterized by significantly different structural features. (C) 2013 Elsevier B.V. All rights reserved.