The release of short-lived isotopes of krypton and xenon may be delayed by passage through an adsorbent bed. Such a process results in the effective removal of these radionuclides if the holdup time is long in comparison with their half-lives. Mechanisms influencing the efficiency of this type of holdup bed include molecular diffusion, eddy diffusion, and mass transfer resistance. At low carrier-gas velocities, molecular diffusion is the controlling factor; at intermediate carrier-gas velocities, eddy diffusion is more important; at high carrier-gas velocities, mass transfer resistance dominates. A procedure described here permits the effect of mass transfer on the removal of each fission-gas isotope to be calculated. If these effects are ignored, the efficiency of a fission-gas holdup bed can be greatly overestimated.