Cryogenic adsorption is effective for the separative recovery of hydrogen isotopes of small concentrations from the bulk helium gas. The authors performed a screening test to find candidate adsorbents for the recovery of hydrogen isotopes from the bulk helium gas at liquid nitrogen temperature. The screening test indicates that a natural mordenite adsorbent has a quite high adsorption capacity for hydrogen under the helium atmosphere. The effect of the ion exchange for the natural mordenite on the adsorption capacity of hydrogen was also investigated using protium and silver as well. With regard to the adsorbent examined in the screening test, the adsorption characteristics of deuterium were also investigated. For the adsorption of deuterium, it was found that the natural mordenite adsorbent have a high adsorption capacity. The isotope effect on the adsorption of hydrogen isotopes on the natural mordenite adsorbent is not large compared with the MS5A adsorbent.