A comprehensive theoretical study of some pyrazolones and their fixed model compounds is presented. The tautomeric equilibrium and acidity constants of these compounds have been investigated both in the gas and aqueous solution by AM1 method. AM1/COSMO solvation method was employed in the case of aqueous solution calculations. The predicted tautomeric equilibrium constants of the compounds exhibiting the tautomeric equilibrium are in good agreement with the existing experimental data indicating that the oxo tautomers are predominant in aqueous solution; whereas the hydroxy tautomers predominate in the gas phase except compound 8. The ratio of the syn and anti conformers in the hydroxy tautomers were also calculated to find out the most stable conformers. (C) 1999 Elsevier Science B.V. All rights reserved.