Electron-transfer kinetics from strong one-electron reductants such as decamethylferrocene and chromium-(II) complexes to singlet oxygen (1O2: 1Δg) were examined using laser flash photolysis in oxygen-saturated deuterated acetonitrile. The formation of decamethylferricenium ion in electron transfer from decamethylferrocene to 1O2 was observed as a transient absorption band at λmax = 780 nm, whereas the formation of O2.- is detected as a transient absorption spectrum of hexyl viologen radical cation (λmax = 600 nm) which is formed by an electron transfer from O2.- to hexyl viologen. The reorganization energy of the electron-transfer reduction of 1O2 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 1O2 in light of the Marcus theory of electron transfer. The solvent-dependent reorganization energy of 1O2 in acetonitrile is smaller than the corresponding value of ground-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 1O2 in an outer-sphere electron-transfer reaction from ferrocene to 1O2 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., O2.- rather than the excited state (1O2).