Car-Parrinello molecular dynamics and acoustic method have been applied to determine the hydration of HCO3- and CO32- ions in aqueous solution. From CPMD simulations the hydration numbers are 5.3 and 8.5 for the HCO3- and CO32- ions, respectively. From speed of sound and density measurements the hydration numbers for HCO3- and CO32- are 5.5 and 10.2, respectively. Our attempt to answer the old question concerning the status of HCO3 in solution quite well support the earlier spectroscopic results that no (HCO3)(2) is present in the solution but the existence of the minor amount of dimers cannot be definitely excluded. (C) 2011 Elsevier B.V. All rights reserved.