The keto-enol tautomerization of 2,4-pentanedione was studied by ab initio molecular orbital and density functional theory calculations. All of the 10 unique conformers (2 for the ketone and 8 for the enol) in the gas phase were located at the HF/6-31G(d), B3LYP/6-31G(d), and MP2/6-31G(d) levels, respectively. Free energy differences between the two minimum structures of ketone and enol in the gas phase were obtained from frequency calculations. The solvation free energies of the two conformers were obtained from the continuum CPCM model. The computed equilibrium constants in different solutions were compared to the experimental literature values.