High-level ab initio calculations were performed on five of the most stable tautomers of 6-thioguanine (6TG) at the second- and fourth-order levels of correlated Moller-Plesset theory using Pople’s type of valence double-zeta basis sets with additional sets of polarization and (in some cases) diffusion functions. At the highest applied level of theory (MP4(SDQ)/6-31++G(d,p)/MP2/6-31G(d,p)+0.9(HF/6-31G(d,p)ZPE)), the order of stability of the tautomers was found to be N9(H)trans > N9(H)cis > N1,7(H) > N1,9(H) > N7(H). When the ZPE corrections were not employed, all calculations utilizing structures minimized at the correlated MP2 level returned a stability order of N1,7(H) > N9(H)trans > N9(H)cis > N1,9(H) much greater than N7(H). Calculations carried out using the self-consistent reaction field (SCRF) model to simulate aqueous solution behavior yielded a stability order (with correction for ZPE) of NB(H)trans > N1,9(H) > N9(H)cis > N1,7(H) much greater than N7(H). CNDO/S calculations on the excited states of 6TG tautomers showed higher oscillator strengths and lower excited-state energies for thiones than for thiols; the latter was confirmed experimentally via UV absorbance and luminescence spectroscopic studies on 6TG and two model compounds for specific tautomers. These experimental spectra confirmed the existence of at least two types of 6TG thione chromophores.