Thin sulfonated poly(ether ether ketone) films swell excessively in water. The extent of water-induced swelling is shown to be correlated with the optical anisotropy of the films, due to two distinct phenomena. Firstly, the optical anisotropy is directly related to the amount of water taken up from the surrounding ambient atmosphere, and thus to amount of water present in the material just prior to swelling. Secondly, the optical anisotropy corresponds to internal stresses in the film that affect the free energy of the film, and thus the potential of the film to swell. The anisotropy vanishes upon sorption of liquid water and returns when the water is desorbed. When the water is completely removed, the film changes from more or less colorless to an intense yellow color that can be attributed to molecular assembly of the aromatic rings in the polymer backbone. The color change is reversible and occurs immediately upon exposure to low humidity. For films prepared in the absence of water, the lack of hydration of the sulfonic acid groups affects the microphase separation behavior of the polymer. This is manifested by an apparent lower propensity to water-induced swelling. The possibility to affect the properties of sulfonated polymer films by varying the hydration state of the polymer during preparation can have important implications for applications of such films.