The photophysical and photochemical properties of electronically excited states of thymine and its water complexes are studied using resonantly enhanced multiphoton ionization spectroscopy. After initial excitation by a nanosecond laser, the excited thymine molecule is trapped in a dark state with a lifetime of tens of nanoseconds. Ionization from this dark state by deep ultraviolet (UV) radiation has a substantially high yield. The lifetime of the dark state is rapidly decreased by adding water molecules. These results indicate that the photostability of our genetic code is not an intrinsic property of the bases themselves. Quenching by solvent water molecules may be the key for the photostability of the DNA bases.