With the specific aim of calculating the acidity equilibrium constant (K-a) of carboxylic acids in aqueous solution we investigated the solute-solvent interactions of these acids and their corresponding anions. The pK(a) (-1gK(a)) values have been calculated using density functional theory (DFT). The polarized continuum model (PCM) is used to describe the solvent. Using these methods, we successfully predicted the pK(a)s of 66 carboxylic acids in aqueous with the average error of 0.5 in pK(a) units. Two different thermodynamic cycles have been studied. The theoretical values are in better agreement with the experimental results for those acids with moderate strength of acidity with the pK(a) value higher than 3. (c) 2006 Elsevier Ltd. All rights reserved.