The accurate pK(a) determinations for some organic acids have been investigated using the combination of the clusters-continuum model with the polarizable continuum solvation model (PCM) The formation of molecular clusters by means of the organic acids wrapped up with water molecules leads to the weakness of the mutual function between the polar solvents and the organic acids, and, hence, the accuracy of pK(a) has been enhanced by using a coherent and well-defined approach without external approximations or experimental data. The calculations are performed at the HF/6-311++G(d, p) level and agreed well with experimental data. (C) 2007 Elsevier B.V. All rights reserved.