AM1 semiempirical calculations, both ΔH and ΔS, were carried out on the enol/enol tautomerism of 25 β-diketones and 8 β-ketoaldehydes. In the first case, δΔG_exp was determined by Hansen (Magn. Reson. Chem.1996, 34, 467) and his values correlate reasonably well with δΔG_calc, δΔG_exp = − 0.23 + 0.29 δΔG_calc, r² = 0.83. The calculated differences in energy are linearly related to some geometrical characteristics of the ketoenol, namely the angles about the substituent on the central carbon. To check if this geometrical dependence is related to the Mills-Nixon effect, parallel AM1 calculations on the tautomerism of β-ketoaldehydes and 3(5),4-disubstituted NH-pyrazoles were carried out confirming the influence of the Mills-Nixon effect on the enol/enol tautomerism of β-dicarbonyl compounds.