Potentially tautomeric pyrazole-3-ones were studied, in order to predict their tautomeric equilibrium constants and pK(a) values, using the semiempirical AM1 COSMO solvation model at the SCF level in aqueous solution. Syn and anti conformers of the hydroxy form were also investigated to find a more stable form. The oxo forms have been found to be more stable than the hydroxy ones. The results obtained from the tautomeric equilibria and acidity calculations are in good agreement with experimental data. (C) 2000 Elsevier Science B.V. All rights reserved.