sElectron propagator methods an applied to the ionization energies of the five most stable tautomers of guanine. Excellent agreement with gas-phase photoelectron spectra is obtained for the amino-oxo form of 7H-guanine. According to ionization energy assignments, both 9H-guanine and its amino-oxy tautomers also may be present in the pas phase. The presence of amino-oxy 7H-guanine, however, is less certain, due to its higher total energy. In all cases, the lowest ionization occurs from a pi level. There are strong correlation effects for higher cationic states. Energy orderings of pi and sigma hole states are different for each of the isomers.