Anions of nucleic acid bases may be formed by trapping lowenergy electrons produced in living cells by high-energy radiation. Recent experiments suggested that single-and double-strand breaks develop in DNA exposed to low-energy electrons. [1] Furthermore, charged nucleobases play a critical role in electron and hole transfer in DNA. [2][3][4] Anionic states of nucleic acid bases have been intensively studied both experimentally and theoretically. On the basis of calculations, guanine is believed to have the lowest electron affinity among the nucleobases. [5][6][7] This is consistent with the results of electron spin resonance experiments on g-irradiated DNA, in which the anion is divided between the pyrimidine but not the purine bases. [8] Gas-phase experiments on guanine are scarce because this nucleic acid base easily decomposes at elevated temperatures. Burrow and co-workers reported a vertical electron affinity of À0.49 eV for guanine and assigned it to a hydroxy (enol) tautomer. [9] We have recently determined-at the coupled cluster level of theory with single, double, and perturbative triple excitations (CCSD(T)) [10] -a negative adiabatic electron affinity (AEA) of À0.49 eV for the canonical tautomer. [11] In contrast to earlier experimental and computational predictions, we demonstrate here that guanine supports anionic tautomers that are adiabatically bound with respect to the neutral canonical tautomer by as much as 8 kcal mol À1 , [12] as calculated at the CCSD(T) level of theory. These