The association constants for Hg2+-complexes involving monomeric (TM) and dimeric (TD) forms of thymine are determined by pH-titration measurements made over a wide pH range 2 < pH < 11. In both systems, the experimental data have been analyzed by a pit-mapping method and the concentration distribution of the various complex species is plotted versus a pH gradient.Because of the tendency of Hg2+to form two bonds in a linear configuration, non-chelate complexes [Hg(TM)+] (log β101 = 10.65) and [Hg(TM)2] (log β102 = 20.70) are formed with the monomer thymine.Photodimers, obtained after irradiation of monomer thymine at 260 nm in frozen water (−10 °C), are the (meso)-cis,synforms and can be complexed by the Hg2+ions in [Hg(TD)] (log β101 = 13.55), [HgH−1(TD)]−(log β1−11 = 6.73), and [HgH−2(TD)]2−(log β1−21 = 1.69) species. [Hg(TD)] complex shows an absorption spectrum shifting towards 280 nm, whereas the free dimer ligand absorbs lower than 240 nm only. An irradiation at 280 nm provides the formation of the [Hg (TM)2] complex and consequently the breaking of the cyclobutane cycle in the dimer molecule.