The equilibrium constant for the keto-enol tautomerism of 3-nitrobutan-2-one K-T = [enol]/[ketone] has been measured in water as 4.57 x 10(-3) (pK(T) = 2.34) by combining the rate constants for ketonisation of the enolate form and pK(a) of the ketone at 25 degrees C. The rates of ketonisation were measured by a rapid kinetic technique and the pK(a) was determined spectrophotometrically and potentiometrically as 5.15. A comparison with 2-butanone and acetone shows a strong influence of the nitro group in enhancing the acidity of the substrate and in stabilizing the enol relative to the keto tautomer. By means of semiempirical AM1 calculations, good correlations were found between the atomic charge on the acidic hydrogens and the pK(a) tin water at 25 degrees C) of both tautomeric forms for a number of simple ketones whose pK(a)s and pK(T)s are available in the literature. The agreement of experimental acidity constants of the enol, pK(a)(EH), th, ketone, pK(a)(KH), and the tautomeric constant, pK(T) with predicted values is satisfactory.