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Fractions of free-radical formation in the electron-transfer (ET) quenching of phosphorescent states were measured. The quenching generated a geminate radical pair with a spin multiplicity different from that of the ground state, followed by a rapid backward electron transfer within the geminate radical pair. Therates of the fast backward electron transfer (kb) are estimated from the free-radical fractions. The energy gap between the radical pair and the reactant pair in the ground state affected the rate of spin inverted electron transfer, kb. In the less exergonic processes (< 1.7 eV), kb increased with the energy gap. However, kb did not increase but began to decrease with theenergy gap in the highly exergonic (> 1.7 eV) ET between Rh (II) and Cr (II) compounds and methoxybenzene cation radicals. The extent of the energy gap dependence of kb in the normal region decreased in the following order: Cr (II)-aromatic …