Catalytic activites were measured by recycled gas-vapour reactor. The reaction was of hydrogen isotope exchange between gas and water vapour. The deuterium gas was fed by gas injector into the hydrogen enriched flow. The mixture was introduced into a saturator, where the gas mixture was saturated at the desired temperature. After saturated gas entering the reactor, the deuterium in the gas mixture was transferred into the water vapour by the catalyst. Depleted gas then was dehumidified, and recycled. The catalytic activity depended on the flow velocity significanty, which indicates the external mass transfer plays a role of overall reaction rate. The catalytic activity decreased with time. This deactivation was modelled and analysed.