Pyridinethiones are important ligand precursors of coordination complexes of therapeutic value. In aqueous solution, pyridinethiones can dimerize and tautomerize to the corresponding thiols. However, the tautomerism of pyridinethi ones, which can impact on therapeutic performance, is yet not fully understood. To resolve this important issue, we have carried out ab initio and DFT calculations to compute the geometries, energies, dipole moments, and NMR, IR, and UV-vis spectroscopic properties of all possible tautomers of pyridinethiones and compared our theoretical results with the existing experimental data. We found that the thione form of the tautomer is dominant for monomers of the pyridinethiones studied here. This work can serve as a reference for exploring other similar organosulfur compounds.