Charge transfer in a Nafion membrane incorporating a trinuclear ruthenium complex. Ru-red ([(NH3)(5)Ru(mu-O)Ru(NH3)(4)(mu-O)Ru(NH3)(5)](6+)), was studied using potential-step chronoamperospectrometry (PSCAS). Reversible oxidation of Ru-red (Ru-III-Ru-IV-Ru-III) to Ru-brown (Ru-IV-Ru-III-Ru-IV) was observed under a potential-step from 0 to 0.5 V vs. SCE in the PSCAS measurement. The charge transport rate by the oxidation of Ru-red to Ru-brown in the membrane depended on the complex concentration and was analyzed by both the physical displacement of the complexes and charge hopping between them. It was found that the charge transport takes place in the membrane by both the physical displacement and charge hopping. The charge transfer distance was analyzed based on a intermolecular distance distribution between the redox centers, and the real charge hopping distance was estimated to be 1.1 nm.