Charge transport (CT) in a Nafion film incorporating [(NH3)(5)Ru(mu-pz)Ru(NH3)(5)](4+) ([Ru-II-Ru-II](4+); pz = pyrazine) was studied using a double potential-step chronoabsorptometry (DPSCA) technique. The oxidative CT by [Ru-II-Ru-II](4+)/[Ru-II-Ru-III](5+) (in a potential step from -0.2 to 0.3 V; first step) was compared with that by [Ru-II-Ru-III](5+)/[Ru-III-Ru-III](6+) (in a successive potential step from 0.3 to 0.7 V; second step). The predominant CTs by a physical displacement mechanism of the complexes were suggested for both of the steps. However, the rate constant of the CT for the first step was 5.2 time higher than that for the second step at 25 degreesC. The results on CT kinetics and dynamics revealed that the CTs in both of the steps are entropy-controlled in the activation at room temperature. The DeltaH(double dagger) (12 kJ mol(-1)) for the physical displacement in the second step was lower than that (24 kJ mol(-1)) for the first step, showing that the physical displacement for the second step is enthalpically favorable compared with that for the first step. The corresponding DeltaS(double dagger) (-234 J K-1 mol(-1)) for the second step is lower than that (DeltaS(double dagger) = -184 J K-1 mol(-1)) for the first step, making the physical displacement for the second step more entropically difficult than that for the first step. The entropy effect on the physical displacement was discussed on the basis of the interaction of the complexes with Nafion and reorganization of the solvent (water) molecule involved in the process. The DeltaS(double dagger) for the physical displacement of redox complexes in the film decreased with the positive charge of the complexes, suggesting that the highly positive-charged complex is entropically unfavorable for physical displacement, presumably due to higher degree of solvation of the complex and the sulfonate groups in their dissociation.