The reversible addition of H2 to metal sites is a fundamental step in catalysis. The importance of the reversibility of this step is perhaps most clearly illustrated in hydrogen-for-deuterium (H/D) exchange reactions. In its simplest form, H/D-exchange involves the selective isotopic labelling of molecules through reaction with D2. Catalysts which promote this type of H/Dexchange must be capable of reversible activation of H2 (and its isotopomers).