The radiationless decay mechanisms of the S1 excited states of the 7H-keto-amino, 7H-enol-amino, and 7Hketo- imino tautomers of guanine have been investigated with the CASPT2//CASSCF method. Out-of-plane deformation of the six-membered ring or the imino group as well as dissociation of NH bonds have been considered as photochemical pathways leading to conical intersections with the electronic ground state. It has been found that all three tautomers can reach S0-S1 conical intersections by out-of-plane deformation. However, only in the 7H-keto-amino tautomer the reaction path leading to the conical intersection is barrierless. This tautomer also has the lowest energy barrier for hydrogen detachment via the 1πσ* state, whose potential energy surface intersects that of the 1ππ* state as well as that of the ground state. The other tautomers of guanine exhibit substantial energy barriers on their S1 potential energy surfaces with respect to both reaction mechanisms. These findings suggest that the 7H-keto-amino tautomer exhibits the shortest excited-state lifetime of the three tautomers due to particularly fast nonradiative deactivation processes through S0-S1 conical intersections. The computational results explain the remarkable observation that the energetically most stable 7H-keto-amino tautomer is missing in the resonant two-photon ionization spectrum of guanine in a supersonic jet. The results also explain that the energetically less stable 7H-enol-amino and 7H-keto-imino tautomers have longer excited-state lifetimes and are thus detectable by resonant two-photon ionization.