We investigated the CO2 adsorption of mesoporous ceria under high humidity. In the presence of co-adsorbed water, mesoporous ceria sustained reversible CO2 adsorption-desorption behavior with a high CO2 uptake, whereas zeolite 13X exhibited almost no CO2 adsorption under these conditions over a wide pressure range. In situ FT-IR data showed that the adsorbed CO2 on the ceria surface was primarily in the form of hydrogen carbonate, and that this state was not changed even after water vapor was introduced.