The kinetics of electron transfer between cationic complexes of Ru-II confined within Nafion(R) coatings on electrodes and anionic oxidants in solution were measured and analyzed according to a model in which the reactants in the Nafion(R) exhibit an array of formal potentials with a Gaussian distribution. The variety of anionic co-reactants employed provided a range of driving forces for the electron-transfer cross-reactions involved. With thermodynamically disfavored cross-reactions the experimental results could be accommodated only by taking into account the array of formal potentials of the mediating complexes within the Nafion(R) coatings. For thermodynamically favored reactions it was possible to approximate the array of formal potentials by a single formal potential at the center of the array. It is shown that the linear correlation between the rate constants and driving forces of electron-transfer cross-reactions predicted by Marcus theory is not affected when one of the co-reactants presents a distributed array of formal potentials instead of a single value. Kinetic data are presented for the reactions of Ru(NH3)(6)(2+) and/or Ru(NH3)(5)py(2+) with Ru-III(edta)(-), Fe-III(edta)(-) and FeXW(11)O(39)(m-) (X = P, m = 4; X = Ge, Si, m = 5).