Abstract: A process for the exchange of hydrogen isotopes between streams of gaseous hydrogen and liquid water, wherein the streams are brought into contact with one another and a catalyst assembly at a temperature in the range 0.degree.-300.degree. C. The catalyst is an inherently hydrophobic material such as polytetrafluoroethylene, polyethylene, polystyrene, polymethacrylate, polypropylene or carbon black carrying a catalytically active metal, for example, platinum, rhodium and nickel. The inherently hydrophobic support allows the catalytically active metal to catalyze the hydrogen isotope exchange reaction between hydrogen gas and water vapor in the presence of liquid water while retarding loss of activity of the catalytically active metal by contact of the metal catalyst with liquid water.