This paper describes the partitioning equilibrium of 1, 1 ' -dibenzyl-4,4 ' -bipyridinium cation (BV2+) between aqueous electrolytes and Nafion films, and the influence of BV2+ -Nafion interaction on the rate of charge transport in loaded films. The voltammetric charge measured at BV2+ -loaded film electrodes demonstrated that the redox ion was highly concentrated in the polyanionic film from its dilute aqueous solution. An electrochemically poor response of highly loaded polymers resulted from the deactivation of the incorporated BV2+, which was due to the dehydration of the polymer. Although hydrophobic interactions contributed to some extent to bind the redox species to the polymer, the reduced positive charge of the redox species weakened the binding. Thus, an electrostatic interaction between them is a major factor determining the incorporation of BV2+ into the polymer. The impedance response of the loaded film electrodes was consistent with the behavior expected for the diffusion of charge carriers through a layer of finite thickness. When the surface concentration of BV2+ increased from 2 to 40 nmol cm (-2), the apparent diffusion coefficient for charge transport decreased by a factor of 14. This dependence indicates a primary role of the physical diffusion of BV2+ in charge transport through the film. A decrease in the BV2+ mobility with increased loading arises probably from the ionic association between incorporated redox ions and polymer sulfonate groups, which are caused by a reduction in the film permittivity induced by cation exchange.