A study of laser kinetic spectroscopy on [Ru-II(tpy)(BL)Co-III(tpy)](5+) (BL: 1,4-bis[2,2':6',2"-terpyridine-4-yl]benzene (btbz) and 6',6"-bis(2-pyridyl)- 2,2:4,4":2",2"-quarter-pyridine (bpqp)) revealed rise-and-decay of a triplet Ru-to-ligand charge-transfer state, (CT)-C-3(Ru), and an electron-transfer product of [(tpy)Ru-III(BL)Co-II(tpy)](5+). (CT)-C-3(RU) underwent electron-transfer with a rate constant of 0.3 x 10(12) and 1 x 10(12) s(-1) to produce [(tpy)Ru-III(BL)Co-II(tpy)](5+) with a doublet electronic configuration d(pi)(6)d(sigma*) of cobalt(II). The return electron transfer (RET) of [Ru-III(tpy)(BL)Co-II(tpy)](5+) (BL:btbz and bpqp) in acetonitrile occurred with a rate constant of 1 x 10(10) and 6 x 10(10) s(-1) at 298 K, respectively, which are much faster than intramolecular RET of [(bpy)(2)Ru(BL)Co(bpy)(2)](5+) from cobalt(II) with a quartet electronic configuration of d(pi)(5)d(sigma*)(2). The magnitude of reorganization energy (0.95 +/- 0.15 eV) and electronic coupling matrix element (0.8 and 2 meV) are evaluated from the independence of RET rate on the temperature by using a small entropy change of RET. The small intramolecular reorganization energy (0-0.3 eV) for Co-2(II)/Co-1(HI) redox process of [(tpy)Ru-II(BL)Co-III(tpy)](5+) (BL: btbz and bpqp). are responsible for the fast RET. The magnitude of the intramolecular reorganization energy is in agreement with those (0.27-0.29 eV) calculated by using a density functional theory.