Kinetics for the thermal and photoinduced electron-transfer oxidation of a series of DNA bases with various oxidants having the known one-electron reduction potentials (E-red) in an aqueous solution at 298 K were examined, and the resulting electron-transfer rate constants (k(et)) were evaluated in light of the free energy relationship of electron transfer to determine the one-electron oxidation potentials (E-ox) of DNA bases and the intrinsic barrier of the electron transfer. Although the E-ox value of GMP at pH 7 is the lowest (1.07 V vs SCE) among the four DNA bases, the highest E-ox value (CMP) is only 0.19 V higher than that of GMP. The selective oxidation of GMP in the thermal electron-transfer oxidation of GMP results from a significant decrease in the pH dependent oxidation potential due to the deprotonation of GMP(center dot+). The one-electron reduced species of the photosensitizer produced by photoinduced electron transfer are observed as the transient absorption spectra when the free energy change of electron transfer is negative. The rate constants of electron-transfer oxidation of the guanine moieties in DNA oligomers with Fe(bpY)(3) (3+) and Ru(bpY)(3)(3+) were also determined using DNA oligomers containing different guanine (G) sequences from I to 10 G. The rate constants of electron-transfer oxidation of the guanine moieties in single- and double-stranded DNA oligomers with Fe(bPY)(3)(2+) and Ru(bpy)(3)(3+) are dependent on the number of sequential guanine molecules as well as on pH.