The kinetics of electron transfer between OS(bpy) 3 3+ incorporated in Nafion® coatings on electrodes and IrCl 6 3− anions in solution were examined by means of rotating disk voltammetry. The cation permselectivity of the Nafion confined the reaction zone to the coating-solution interface. As a result, reverse cation exchange, which causes small quantities of Os(bpy) 3 3+ 2+ to be lost from the outermost portions of the coatings, results in large decreases in the rate of the electron transfer reaction. Low concentrations of Os(bpy) 3 2+ were therefore added to the solutions in order to avoid significant losses from reverse ion exchange. Changes in the Donnan potential produced by changes in the concentration of supporting electrolyte were not reflected in the rates of the electron transfer reaction. The rates appeared to be sensitive only to changes in the formal potential of the IrCl 6 2− 3− couple in solution. It is speculated that the potential experienced by the two reactants when they are close enough together for electron transfer to occur is not altered significantly by changes in the concentration of supporting electrolyte.