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 p K a (−lg K 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 p K as of 66 carboxylic acids in aqueous with the average error of 0.5 in p K 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 p K a value higher than 3.