Palladium and molecular sieve adsorbents are promising materials for hydrogen isotope separation judged from previous experimental data of the isotope separation factor, the absorption or adsorption rate constant and the height equivalent to a theoretical plate (HETP). In the present study, the hydrogen absorption or adsorption rates of twin beds packed, respectively, with alumina-supported Pd pellets and molecular sieve (MS-SA) pellets are experimentally determined as a basic study for developing a new type of hydrogen isotope separation system. Very narrow mass-transfer zones compared to the column length move through either of the two columns because of high absorption or adsorption rates. The hydrogen pressure in the zone decreases from the inlet pressure to zero until the front of the zone reaches the column end. The amount of hydrogen absorbed or adsorbed in the zone is almost independent of the hydrogen flow rate. The twin-bed system with an alternately counter-current flow for a semi-continuous isotope separation process is found to be operated effectively by heating-cooling cycles without any external pump. (C) 1999 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved.