The pKa for H2CO3 have been calculated using the combination of the extended clusters-continuum model (ECM) with the polarizable continuum solvation model (PCM) and a comparison has been carried on with the pKa prediction of HCOOH. Our results show that the calculated and experimental pKa of HCOOH are in good agreement as two water molecules are added, the best agreement of pKa(1) for H2CO3 is for n = 3 ( the number of water molecules) and the closest agreement of pKa(2) is for n = 4 at B3LYP/6-311++G(d, p) level. On the base of the results, attempts have been made to evaluate the hydration numbers. (C) 2008 Elsevier B.V. All rights reserved.