The diffusion coefficient (D-PV2+) and the concentration (C-PV2+(a)) of the actually electroactive PV2+ in the poly(ethylene oxide) (PEO, average molecular weight : 400) media containing 0.1 M LiClO4 and various concentrations of propyl viologen dibromide (PVBr2) could be separately estimated without previous knowledge of either, based on a combined use of steady-state cyclic voltammetry at carbon fiber electrodes (diameter : 9 pm) and potential-step chronoamperometry at glassy carbon electrodes (diameter : 1 mm) as a function of PVBr2 concentration (C-PV2+ = 2-20 mM) and temperature (10-60 degrees C). Both C-PV2+(a) and D-PV2+ significantly varied with C-PV2+, C-PV2+ values were smaller than C-PV2+, indicating that all of the dissolved PVBr2 are not actually electroactive. D-PV2+ decreased with increasing C-PV2+ (and C-PV2+(a)), and the charge transport in the PEO matrix was thus found to occur not via the electron-hopping process between PV2+ and PV+ (the monocation state of PV2+), but via the physical diffusion of PV2+, Further, it became apparent that at C-PV2+ > 5 mM the temperature dependence of D-PV2+ is not the Arrhenius type and follows the VTF equation, that is, PV2+ ions are transported along with the segmental motion of the PEO chains. Based on the DSC measurements of the PEO-LiClO4 (0.1 M)-PVBr2 complexes, their glass transition temperatures were found to decrease with increasing C-PV2+, suggesting that PVBr2 functions as a plasticizer.