In disagreement with an intuitive prediction on the basis of the molecular size and dipole moment, it is observed empirically that the solubility of carbon dioxide (CO2) in water is larger than that of carbon monoxide (CO). In order to shed light on this puzzling behavior, the solvation free energy of CO and CO2 in aqueous solution is studied by means of the RISM-SCF/MCSCF method, a combined ab initio molecular orbital theory and statistical mechanics theory of molecular liquids. It is shown that the specific hydrogen bonding between oxygen atoms in CO2 and water molecules makes CO2 more soluble.