Single crystal X-ray structure determinations of six crystals 1–6 of β-diketones, the related DFT calculations as well as a systematic investigation, on the CSD (Cambridge Structural Database) files, of all acyclic β-diketones having at least one α-hydrogen, in both β-diketo and β-keto–enol tautomeric forms, are reported. In spite of the stabilization energy gained by the formation of strong intramolecular O–H⋯O resonace assisted hydrogen bonds (RAHB) a certain number of non-enolized structures were retrieved. The structural data show that the steric and electronic properties of the substituents play a definite role in tuning the hydrogen bond strength and determining the enolic site but the driving force able to shift from the more common β-keto–enol tautomer to the β-diketo one can be only the steric hindrance of bulky groups. In this context the substituents in position 2 play a crucial role in establishing the tautomeric form. In fact, while the 2-unsubstituted β-diketones (or 2-substitued by a group linked by a sp²carbon) assume almost exclusively the β-keto–enol form with some exceptions for very bulky substituents, β-diketones carrying 2-alkyl substituents, in general, display the β-diketo tautomeric form. The only exceptions are the 2-alkyl curcumin derivatives where the planar β-keto–enol group is stabilized by extended π-conjugation within the whole molecule and by the absence of short contacts between the alkyl R₂groups and R₁ or R₃ substituents. DFT calculations on the six compounds, 1–6, show that in the four more overcrowded structures, 3–6, the trans-β-diketo tautomer is more stable than the Z-β-keto–enol isomer unlike what happens for 1 and 2 where the Z-β-keto–enol isomer is the most stable by a few kcal mol⁻¹. Thereby, the occurrence of the trans-β-diketo tautomer for all compounds, in the crystal, can be interpreted in terms of the existence of a large activation energy in the mechanism to attain the Z-β-keto–enol isomer containing an intramolecular O–H⋯O hydrogen bond.