Abstract: The separation of isotopes, 235U(Ⅳ)/238U(Ⅳ)and 140Ce(Ⅲ)/142Ce(Ⅲ) with dicyclohexyl-l8-crown-6 and cyclohexyl-15-crown-5 in 1, 2-dichlo-roethane is studied. The single stage separation factors obtained are: α u(DC18C6)= 1 .0007±0. 0001, α ce(DC18C6)= 1 .00 11±0 .000 3 . α ce(CH15C5) = 1 .0008 ±0.0002.These results seem to indicate that for a chemical exchange system containing macrocyclic polyether in the organic phase ,relatively large isotope effect can be obtained even for isotopes of heavy elements such as lanthanides and actinides.