The properties of ion-containing polymers with tethered sulfonic acid groups determine the performance of solid-state electrochemical devices such as fuel cells and electrolyzers. Despite significant research on bulk membranes (thicknesses of 10 m or greater) of ion-containing polymers, little attention has been given to the properties of these materials when confined in thin films with thicknesses less than 100 nm. Ion-containing polymer thin films in porous electrodes are responsible for transporting ions, reactants, and water to and from the reaction sites and the properties of the thin polymer films that sit directly on top of the catalytic reaction sites may be a critical bottleneck in device performance. Thin films of polymers are known to have altered properties, such as glass transition temperature and modulus, due to confined and free surface effects. We have used fluorescence spectroscopy to measure the confinement of water in thin ion-containing polymer films. Antiplasticization, or a stiffening of the polymer thin film was observed at low levels of hydration as determined by an increase in fluorescence intensity of a rotor dye. As the film was hydrated further, the fluorescent intensity decreased, which is indicative of increased dye rotation and plasticization of the polymer thin film. The extent of antiplasticization varied depending on whether the film was in contact with a silicon native oxide surface or a gold surface. Spectroscopic ellipsometry studies revealed that the refractive index of the thin film decreased with a decrease in film thickness on both Si-native oxide and Au. Mueller-matrix variable angle spectroscopic ellipsometry was employed to measure the birefringence of the thin films and the optical properties were correlated with observed changes in water uptake and plasticization.