Effect of co-existing H2O (isotopic swamping effect) in the adsorption of tritiated water vapor on molecular sieves was experimentally studied by two means. One was by breakthrough experiment. It was found that H2O-HTO two components Langmuir type adsorption isotherm is satisfactory to explain the HTO breakthrough curve. The other was by using a divisible column, where behavior of HTO at much lower concentration than at the inlet and the axial distribution of tritiated water on the adsorbent were studied. Concentration of tritiated water vapor at the column outlet was found to decrease with the increase of H2O concentration of the input. However the improvement of decontamination factor was little.