High-level quantum chemical calculation methods have been applied to explore the influences of the oxygen at the C8 position on the intramolecular proton transfer between the B-oxo and B-hydroxy forms of C8-oxidative guanine. The predicted order of relative stability of the tautomers of C8-oxidative guanine in the gas phase C8-OG1, C8-OG1* > C8-OG2, C8-OG2* is somewhat different from the stability of monohydrated C8-oxidative guanine: C8-OG1 . H2O > C8-OG1*. H2O > C8-OG2 . H2O > C8-OG2*. H2O. The inclusion of quantum mechanical tunneling in the calculation dramatically increases the proton-transfer rate in C8-oxidative guanine. The tunneling rates were evaluated to be 10(-2) s(-1) for the gas phase and 10(9)-10(10) s(-1) for the water-assisted process. Our results suggest the importance of the tunneling effect and that it might dominate the tautomeric process in guanine and its derivatives at room temperature.