Redox reaction and charge transport of trinuclear ruthenium complex, Ru-red ([(NH3)(5)Ru(mu-O)Ru(NH3)(4)(mu-O)Ru(NH3)(5)](6+)) were investigated in a Nafion (Nf) membrane as well as in a homogeneous aqueous solution using a potential-step chronoamperospectrometry (PSCAS) in addition to a cyclic voltammetry (CV). It has been shown in CV that Ru-red is oxidized to Ru-brown and then oxidatively transforms to stable redox species in the membrane during repetitive cyclic scans. In a potential-step from 0 to 0.5 V (vs SCE) in PSCAS, a reversible oxidation of Ru-red to Ru-brown was observed in both the membrane and solution. The charge transport based on Ru-red/Ru-brown took place in the membrane by both physical displacement of the complex and charge hopping between them, while in the aqueous solution charge was transported only by physical displacement of the complex. In a potential-step from 0.5 to 1.5 V (vs SCE); an irreversible oxidative transformation of Ru-brown takes place to produce the species which has almost no absorption in the visible region. The transformed species were stable and redox active in the membrane.