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Gadolinium isotopes were fractionated in a liquid-liquid extraction system using dicyclohexano-18-crown-6.The isotope enrichment factors showed breakdowns of the conventional Bigeleisen-Mayer approximation for every condition of the initial phases; the concentrations of gadolinium and hydrochloric acid and the type of organic solvents. The nuclear mass effect and the field shift effect were estimated by use of the isotope pairs 152Gd_I60Gd and 154Gd_16°Gd. Since both isotopes.P'Gd and 154Gd, have the unique characteristics of the nuclear charge radii, 's, the unusual isotope effects due to the field shifts took place, while the mass effects of the other isotopes canceled the field shift effects. The maximum isotope effect was observed to be 6160.152 = 0.0252 ± 0.0024 whose initial aqueous phase was 0.023 M GdCl, in 12 M HCl, and its organic phase was 0.2 M DC18C6 in chloroform: this was 0.00315 ± 0.00030 in terms of the enrichment factor for unit mass. 1219

Chromium isotopes were fractionated by the liquid-liquid extraction system using dicyclohexano-18-crown-6. We found a mass-independent isotope effect that the intermediate isotope of 52 Cr was enriched in the organic phase. Because of its magic number neutrons N ) 28, 52 Cr has the smallest nuclear charge distribution in naturally occurring chromium isotopes. The enrichment property gives the confident experimental fact of the nuclear field shift effect in the new Bigeleisen theory.