A study of the metal-to-metal charge-transfer transition within the binuclear complex pentaammineruthenium(III)(mu -cyano)pentacyanoiron(II) (NH3)(5)RUIII-NC-Fe-II(CN)(5)(-), was carried out in salt solutions, water-cosolvent mixtures, and micellar solutions containing sodium dodecyl sulfate and hexadecyltrimethylammonium chloride (CTACl): Using these data, as well as the reaction free energies (obtained from electrochemical measurements), the rate constants, k(et), for the forward and reverse electron-transfer processes have been estimated and compared with data for this and related electron-transfer processes existing in the literature for electrolyte and water-cosolvent solutions. The approximations involved in the method of estimating electron-transfer rate constants are discussed. In the case of micellar solutions, the reorganization energies and driving forces were obtained. The results in these microheterogeneous systems are interpreted taking into account the long-range (Colulombic) interactions between the mixed valence complex and the micellar electric field, along with the short-range electric field derived effects, the latter coming through the dielectric saturation phenomenom produced by the micellar electric field on the solvent surrounding the binuclear complex, when it is near the micellar (CTACl) surface.