We performed a theoretical investigation, at the CC2/aug-cc-pVDZ level, of the ring-deformation mechanisms of four guanine tautomers (oxo, hydroxy, N9H, and N7H). The study showed that the optimized conical intersections S₀/S₁ are accessible through the (1)ππ* excited states of tautomers. The optimized conical intersections S0/S1, which show deformation at the pyrimidine ring, have high energies. This means that the relaxations of the (1)ππ* excited states via internal conversion are disfavored. For two tautomers we found crossing points (1)ππ*/(1)πσ* of the excited-state reaction paths, revealing the possibility of a population of the (1)πσ* excited state by the (1)ππ* excited state.