The prototropic, tautomeric, and ionization equilibria of all of the structures of 2-hydroxy-3-(3-oxo-1-phenylbutyl)chromen-4-one (warfarin) were studied experimentally and theoretically The structure elucidation and acidity constant determination aim; with the tautomeric equilibrium constants were carried out experimentally by a UV-visible spectroscopic method and theoretically investigated at the HF/6-31G(d) and B3LYP/6-31G(d) levels of theory. The obtained K values were evaluated by structure elucidation and a protonation mechanism Using the gauge-including atomic-orbital (GIAO) method for calculating H and (13)C nuclear magnetic shielding tensors, the Hartree-Fock (HF) and density functional (DFT) levels of theory were applied to warfarin Geometry optimizations were also performed on each species, and the absence of negative frequencies Veil fled that all structures were true minima Theoretical values were compared to the experimental data Satisfactory agreement between the experimental chemical shifts and the theoretical values of shielding constants (calculated by GIAO/DFT and GIAO/HF methods) was obtained.