Fluorescence and absorption spectra of 10(-4) M thymine in aqueous solutions at different pH values are reported at room temperature. The shape of the fluorescence spectrum is found to change when the excitation wavelength is varied. The absorption spectra for the aqueous pH 4-8 thymine solutions in the range 240-300 nm are believed to include the diketo tautomer T1 and the enol-keto tautomer T2. Steady-state fluorescence spectra and ab initio calculations support the existence of these neutral species in their ground state. The lifetimes of the excited states of the T1 and T2 tautomers are 0.72 and 3.87 ns, respectively. Aqueous thymine solutions with pH values below pH 3 contain a protonated enol-keto form of T2 in addition to the major diketotautomer T1 in its neutral form. The lifetime of the protonated enol-keto form is 4.2 ns. On the other hand, thymine solutions with pH values above pH 8 contain mixtures of the neutral forms of T1 and T2 as well as their monoanioic forms. However, aqueous thymine solutions at pH values above pH 11 contain a 1:1 equilibrium mixture of the monoanionic forms 1-HT- and 3-HT-. The lifetimes for these monoanionic forms are expected to be shorter than 0.05 ns.