The oxidation of guanine (G) is studied by using transient absorption and time-resolved resonance Raman spectroscopies combined with pulse radiolysis. The transient absorption spectral change demonstrates that the neutral radical of G (G(center dot)( -H+)), generated by the deprotonation of G radical cation (G(center dot+)), is rapidly converted to other G radical species. The formation of this species shows the pH dependence, suggesting that it is the G radical cation (G(center dot+))' formed from the protonation at the N7 of G(center dot)( -H+). On one hand, most Raman bands of (G(center dot+))' are up-shifted relative to those of G, indicating the increase in the bonding order of pyrimidine (Pyr) and imidazole rings. The (G(center dot+))' exhibits the characteristic CO stretching mode at similar to 1266 cm(-1) corresponding to a C-O single bond, indicating that the unpaired electron in (G(center dot+))' is localized on the oxygen of the Pyr ring.