Charge transfer between Ru(bpy):(2+)(3) (where bpy is 2,2’-bipyridine) complexes incorporated into a Nafion(R) membrane has been investigated using in situ spectrocyclic voltammetry and statistical calculation of the intermolecular distance between the complexes. The membrane was prepared either by a mixture casting method in which an alcoholic mixture solution of Nafion and the complex was cast to form a membrane, or by an adsorption method in which the complex was adsorbed from its aqueous solution into a precoated Nafion(R) membrane. It has been found that the electrochemical reactivity of the complex in a membrane prepared by the adsorption method is much higher than that prepared by the mixture casting method. The charge transfer distance between the complexes (R(0)) in the membrane prepared by the adsorption method was 1.6 nm (scan rate, 2 mV s(-1); pH 5.5), when assuming a random dispersion of the complex, which is significantly longer than that (1.1 nm) in the membrane prepared by the mixture casting method. The emission lifetime of the photoexcited Ru(bpy):(2+)(3) in the membrane suggests that the complex is localized in the membrane when it is prepared by an adsorption method; such localization would make the calculated R(0) value greater than the real value. The fraction of the volume in the membrane where the complex is adsorbed was estimated as 20 +/- 3%, by comparing the real local complex concentration in the membrane obtained from the R(0) value (1.1 nm) for the mixture casting system with the apparent average complex concentration.