Apparent formal potentials are measured for Os(bpy)(3)(3+/2+) exchanged in Nafion films on electrodes. The apparent formal potentials in the films are compared with the formal potential for Os(bpy)(3)(3+/2+) in solution using a description that assumes Os(bpy)(3)(2+) and Os(bpy)(3)(3+) equilibrate with sodium ions in the bathing electrolyte solution. Good agreement is obtained between the experimentally determined values of the apparent formal potentials and the values expected using the equilibrium model. Apparent reaction entropies are also measured for Os(bpy)(3)(3+/2+) exchanged in Nafion. Once the entropy of ion transport is taken into account, it appears that the reaction entropy of Os(bpy)(3)(3+/2+) in Nafion is similar to that observed in aqueous solutions. The environment in the vicinity of Os(bpy)(3)(3+) in Nafion appears to be very similar to the environment in the vicinity of Os(bpy)(3)(2+) in Nafion. The results also demonstrate that there is a large entropic contribution to the free energy of exchange, which is consistent with the so-called hydrophobic effects observed by other workers.