We explore the effects of hydration on the vibrational spectra of cation-exchanged Nafion thin films. In addition to the initial H+ form of the membrane, the cations selected for study include: Na+, NH4+ tetramethylammonium, tetrabutylammonium, hexadecyltrimethylammonium, and tris(2,2'-bipyridine)osmium(II). A flow cell is used to follow the changes occurring in a 1000-Angstrom film in response to variations in the humidity of a nitrogen purge gas flowing through the cell. The structural changes occurring due to the binding of water to these cation-exchanged membranes is followed across a broad range of water contents with IR difference spectroscopy. Our results demonstrate a complex role for water in mediating the polyanion-cation interactions. Side-chain reconstructions may play an important role in the ion-binding and transport properties of the ionomer domains.