Concentration of tritium and/or deuterium that is a contaminant in H.sub.2 O, followed by separation of the concentrate from the H.sub.2 O. Employed are certain metal oxo complexes, preferably with a metal from Group VIII. For instance, [Ru.sup.IV (2,2',6',2"-terpyridine)(2,2'-bipyridine)(O)](ClO.sub.4).sub.2 is very suitable.
Funding was provided by the Environmental Management Science Program, Office of Environmental Management, United States Department of Energy, under Contract No. OR17SP22 3TFW. Therefore, the United States Government has certain rights in the invention.