AbstractA recycle reactor system has been developed to measure the activities of catalysts for isotope exchange between hydrogen gas and water vapour. To allow testing of reasonably large quantities of catalysts, the reactor was operated with a high recirculating flow of hydrogen gas passing through a saturator to provide the water vapour required for the reaction. In this mode of operation, only a small spiking gas flow was required as feed compared to the very high feed flow that would be required for once‐through operation. The reactor was also operated as a trickle bed by recirculating the saturator water. By testing catalysts under both vapour and trickle bed modes, it was possible to investigate the effect of trickling water over the catalyst on the catalytic activity.Kinetic exchange rates under vapour phase operation were calculated from both plug flow and well‐mixed reactor models. The former model was found to be the appropriate one for the ranges of operating conditions investigated. However, for trickle bed operation, the well‐mixed reactor model was found to be the suitable one.Vapour phase and trickle bed tests done with random bed and structured bed catalysts indicated that the gas phase isotope exchange reaction was not impaired by the presence of liquid water in the re