The sensitivity of the electrochemical responses obtained from electroactive counterions incorporated in Nafion coatings to the extent of hydration of the coatings(27-33) is shown to be enhanced significantly when the electroactive ion is the only counterion present in the coating. Nafion coatings fully saturated with Os(bpy)(3)(2+) exhibit unusually narrow and sharply peaked anodic currents in first-scan oxidative voltammograms. This voltammetric feature is accompanied by the expulsion of one-third of the Os complex from the coating. The counterion actually ejected is primarily Os(bpy)(3)(3+). As the Os(bpy)(3)(3+) counterions are ejected, H2O molecules enter the coatings. The quantity of H2O molecules incorporated is so large that the mass of the coating (monitored with a quartz crystal microbalance) increases despite the ejection of the heavy Os(bpy)(3)(3+) cations. After several voltammetric cycles, normally shaped voltammograms are obtained that are relatively insensitive to the initial state of hydration of the coatings. The contrasting behaviors of hydrated and unhydrated coatings are compared, and possible explanations are offered for the differences observed.