The electronic structures of the ground and low-lying excited states of thymine and the anions 1-HT− and 3-HT−, derived by removing a proton from nitrogen atom N3 and N1, respectively, have been described by all-electron all-integral LCAO–SCF calculations using Gaussian basis functions. In accord with experiment, we find the carbonyl bonds weakened in the anions relative to thymine because of a decrease in their overlap charge densities. On the other hand, our calculations and interpretation of the observed optical and ESR spectra suggest that both for T and 1-HT− the lowest singlet and triplet π → π* states are very similar and localized in the region of the ethylenic bond of thymine.