The quenching of the luminescent excited state of tris(2, 2'-bipyridine)ruthenium(II) by the complexes Co(NH3)5Ln+ (L = pyridine (py), l, 2-bis(4-pyridyl)ethane (NC5-CH2CH2-C5N), 4,4'-bipyridine (Ne5-C5N), l-methyl-4, 4'-bipyridyl (NC5-C5NCH3+), 1, 2-bis(4-pyridyl)ethene (NC5-CH=CH-C5N)) has been studied at 25 °C, 5.6, and ionic strength 0.10 M (LiC1O4). The quenching constants in the order given above are (1.72 ± 0.21) × 108, (2.02 ± 0.25) × 108, (5.67 ± 0.60) · 108, (1.09 ± 0.01) · 109, and (1.15 ± 0.08) · 109 M-1 s-1, respectively. A parallel series of quenching measurements has been conducted for the free ligands but only NC5-C5NH+, HNC5-C5NH2+, NC5-C5NCH3+, HNC5-C5NCH32+, and the protonated forms of NC5-CH=CH-C5N are effective quenchers, thate constants, in the order given above, being (5.41 ± 0.41) · 108, (1.65 ± 0.29) · 109, (7.92 ± 0.12) · 108, and (2.51 ± 0.02) · 109, for the first four ligands. Flash photolysis experiments with Ru(bpy)32+ show the formation of radicals derived by one-electron reduction of NC5-C5NH+, HNC5-C5NH2+, and NC5-C5NCH3+ and their subsequent disappearance by reaction with Ru(bpy)33+. Flash-photolysis experiments with Ru(bpy)32+ in the presence of the cobalt(III) complexes failed to reveal the existence of analogous radical intermediates. Quantum yield measurements for the production of Co2+ in ru(bpy)32+-Co(NH3)5NC5-C5N3+ and -Co(NH3)5NC5-C5NCH34+ systems show that a Co2+ ion is produced for every quenching event. The results are interpreted in terms of a quenching mechanism involving electron transfer from the excited state *Ru(bpy)32+ to the cobalt(III) complex. For the complexes with 4,4'-bipyridine, l-methyl-4,4'-bipyridyl, and l, 2-bis(4-pyridyl)ethene the detailed electron-transfer quenching is postulated to proceed via a chemical mechanism, whereby the electron is transferred to the pyridine ligand and, in a subsequent step, from the bound radical to the cobalt(III) center.