It is known that the solvation effect plays a crucial role in a ubiquitous reaction, CO2 + OH- -> HCO3-. There is no barrier in the gas phase whereas the barrier exists in aqueous solution. We study the origin of the barrier by using the RISM-SCF method, which provides both of quantum chemical and statistical mechanical information on the solution system from the first principle. The solvation structure and free energy are discussed from the view points of atomic and spatial contributions in the vicinity of the solute molecule. The result is also compared with the dielectric continuum model. (C) 2007 Elsevier B.V. All rights reserved.