Off gases produced in the reprocessing of spent nuclear fuel contain various radioactive components and emission of these components to the environment must be prevented as low as possible. Carbon-14 with a long half-life, which is mainly released as the form of carbon dioxide, is one of such gaseous radioactive materials. One of the measures to capture radioactive gases from the off-gas is the utilization of adsorption technique. In this work, the adsorption behavior of carbon dioxide on synthetic zeolites and natural mordenites was studied at low er partial pressure ranges, Moreover, the authors quantitatively investigated the influence of coexistent water vapor, which is also contained in the off-gas, on the adsorption behavior of carbon dioxide.