The categorization of Nafion side-chain IR group modes by exchange site local symmetry enables correlation of exchange site structure to state-of-hydration and ion exchange. During dehydration the Nafion-[H] exchange site transitions from the sulfonate to sulfonic acid form, corresponding to C-3V and C-1 symmetry, respectively: C-3V,C-HF (1061 cm(-1)) and C-3V,C-LF (969 cm(-1)) bands transition to C-1,C-HF (1414 cm(-1)) and C-1,C-LF (910 cm(-1)) bands. Metal ions (M) with Delta H-hyd below 550 kJ/mol alter the exchange site at all states-of-hydration. They bind with C-3V symmetry and exhibit high orbital overlap with the sulfonate sulfur and oxygen atoms. The Nafion-[H] C-3V,C-LF (primarily sulfonate) vanishes at full dehydration with retention of the C-3V,C-HF (primarily ether link). Theoretical Nafion-[M] C-3V,C-LF bands (e.g., 940 cm(-1) for Li+) are not experimentally observed. Hydration waters of ions with Delta H-hyd > 1800 kJ/mol cannot be displaced by sulfonate oxygens. Thus, the Nafion-[H] C-3V,C-LF persists at all states of hydration. (C) 2015 Elsevier Ltd. All rights reserved.