Charge transport (CT) in a Nafion membrane containing mu-oxobis[aquabis(2.2'-bipyridine)ruthenium(III)] complex. [(bpy)(2)(H2O)Ru-O-Ru(H2O)(bpy)(2)](4+) (bpy = 2.2'-bipyridine. abbreviated to Ru-III -O-Ru-III) was investigated by potential-step chronocoulospectrometry (PSCCS). Electrochemical reduction of Ru-III-O-Ru-III in the membrane occurred irreversibly to form [Ru(bpy)(2)(OH2)(2)](2+) monomer. The CT by reduction of Ru-III-O-Ru-III. in the membrane was suggested to take place by physical displacement of the complex, which is quite different from the mechanism in the CT by oxidation of Ru-III-O-Ru-III in the same membrane in which charge is transported by charge hopping based on reversible redox reaction between Ru-III-O-Ru-III and Ru-III-O-Ru-IV. The fractions of the electrochemically reacted complex in the membrane for the oxidative CT was dependent on the complex concentration, and the yield was low (maximum fraction = 0.42 at 0.87 M) relative to the reductive CT. By contrast. the fraction for the reductive CT was independent of the concentration over 0.12 M and close to unity. The different concentration dependence of the fraction was discussed related to the difference in the CT mechanism.