The oxidation of Ru(NH3)(6)(3+) by Os(bpy)(3)(3+) is a very slow process in aqueous acid. The rate increases with pH but the reaction remains slow at pH 7. However, when the two reactants are incorporated in Nafion coatings on electrodes the oxidation can proceed much more rapidly, especially with Nafion coatings that are not fully hydrated. Rotating disk and cyclic voltammetry were employed to determine the stoichiometry of the reaction and to measure its kinetics both in solution and in Nafion coatings. The Ru(NH3)(6)(3+) is oxidized to Ru(VII) at a rate that is first order in Ru(NH3)(6)(3+) and Os(bpy)(3)(3+) and inverse first order in H+. The reaction rate constant evaluated in solution was similar to that evaluated in Nafion, The higher reaction rate obtained when the reactants are incorporated in Nafion results from their higher concentrations, not from significant enhancement of their reactivities. The higher reaction rate obtained from partially dehydrated Nafion coatings is believed to result from a breakdown in cation permselectivity that permits the ionic current inside the coating during anodic current flow to be carried by anions instead of by the ejection of Ru(NH3)(6)(3+) cations from the Nafion.