The resultant effect of two post-casting treatments on the anion-rejection characteristics and adherence to a smooth Pt electrode of a perfluorosulfonate ionomer (Nafion) cast as a thin film (4 μm) from the commercially available alcohol solution is reported. The cyclic voltammetry of ferricyanide from both an acidic and neutral pH solution (2 mM sodium ferricyanide +0.2 M sulfuric acid and 50 mM sodium ferricyanide +0.2 M sodium sulfate, respectively) is used as the measure of anion rejection. Unannealed films which are cured at room temperature serve as the reference and are most permeable to ferricyanide in comparison to high-temperature (170 °C), vacuum-annealed films (20mm Hg) or films which are cured in a low-humidity environment at room temperature under 20mm Hg vacuum (drycured film). For example, after seventy hours of soaking in the neutral pH solution, the amount of ferricyanide absorbed in the unannealed film increased 390% above that after one hour. The dry-cured film, in comparison, absorbed less ferricyanide after one hour; but after seventy hours, even though the amount absorbed was still less than that in the unannealed film, the increase was 1300% above the amount after one hour. In contrast, the thermal-annealed film showed no measurable absorption of ferricyanide even after seventy hours of exposure. All films absorbed ferricyanide from the acidic solution but the thermal-annealed film exhibited the lowest amount. Thermal annealing results in the most stable film of the three: After 16 hours of exposure to 6 M sulfuric acid, the thermal-annealed film adhered to the platinum disk upon rotation, but the other two were sheared off the surface.