The novel -diketo compound (3-acetyl-4-oxopentanoic acid) OPAA is here synthesized, completely characterized in the solid state by means of X-ray crystallography and in solution by potentiometry and 1H and 13C NMR spectroscopy. In the solid state, OPAA exhibits the di-keto (DK) structure, while in solution a strongly solvent dependent tautomeric equilibrium is observed. Theoretical ab-initio calculations employing DFT at B3LYP/6-311G** level and different methods of theoretical model chemistry (CBS-4M, G3MP2, CBS-QB3) are used to extensively investigate the tautomeric equilibrium in comparison with experimental data. Solvent effects are evaluated using CPCM continuum solvation method; among all implied methods, CBS-4M is the one that better predicts experimental data and is able to qualitatively describe tautomeric equilibrium in solution, allowing thermodynamic calculation of pKa. Furthermore a supermolecular solvent approach is used to better analyze solvent-solute interactions in order to forecast chemical properties.