The dinuclear complex ion {(mu-bpym)[Ru(NH3)(4)](2)}(4+), isolated as the tetrakis(hexafluorophosphate) salt, undergoes two reversible oxidation and two reversible reduction processes in dry acetonitrile/0.1 M n-Bu4NPF6. The more accessible 3+, 4+, and 5+ states were characterized spectroelectrochemically (UV/vis/NIR). The EPR-active 3+ form (g(1) = 2.0089, g(2) = 1.9967, g = 1.9308, [S] = 1.9785) is best described as a complex between the bpym radical anion and two tetraammineruthenium(II) fragments. The 5+ state, on the other hand, is EPR silent down to 3.5 K due to rapid relaxation involving metal-bared orbitals; it exhibits a weak absorption in the 3200 cm(-1) region which may be associated with the intervalence-transfer (IT) transition. Although the question of (de)localization in the mixed-valent 5+ species cannot be answered from the available data, the comproportionation constant of 10(5.1) signifies strong metal-metal interaction under these conditions.