Electron-transfer kinetics from strong one-electron reductants such as decamethylferrocene and chromium(II) complexes to singlet oxygen (O-1(2): (1)Delta(g)) were examined using laser flash photolysis in oxygen-saturated deuterated acetonitrile. The formation of decamethylferricenium ion in electron transfer from decamethylferrocene to O-1(2) was observed as a transient absorption band at lambda(max) = 780 nm, whereas the formation of O-2(.-) is detected as a transient absorption spectrum of hexyl viologen radical cation (lambda(max) = 600 nm) which is formed by an electron transfer from O-2(.-) to hexyl viologen. The reorganization energy of the electron-transfer reduction of O-1(2) in acetonitrile has been determined as 1.73 +/- 0.14 eV from the analysis of the rate constants of electron transfer from the one-electron reductants to 102 in light of the Marcus theory of electron transfer. The solvent-dependent reorganization energy of O-1(2) in acetonitrile is smaller than the corresponding value of around-state oxygen in water (1.97 +/- 0.03 eV), reflecting the longer O-O bond length in the excited state in reference to the ground state and the smaller solvation in acetonitrile. The reactivity Of O-1(2) in an outer-sphere electron-transfer reaction from ferrocene to O-1(2) is enhanced by the addition of scandium triflate [Sc(OTf)(3)]. This is ascribed to the strong binding Of Sc3+ with the one-electron reduced species, i.e., O-2(.-) rather than the excited state (O-1(2)).