Abstract There are large differences between the acidity of the enol of the acyclic diketone, 2,4-pentanedione and those of two cyclic diketones, 1,3-cyclopentanedione and 1,3-cyclohexanedione. Computational studies have demonstrated that these differences are largely due to the strength of the internal low-barrier hydrogen bond (LBHB) in the enol of 2,4-pentanedione. It is thus estimated that the lower limit of the additional free energy of formation in water for this LBHB over that of a conventional hydrogen bond is 4.1–5.3 kcal mol −1 .