JOHNSON AND NIER1 have measured the atomic masses of some of the rare-earth isotopes and have shown that the mass difference cerium-142-(barium-138 + helium-4) is equivalent to 1.68 ?? 0.10 MeV. Similar results for the naturally occurring samarium and neodymium isotopes show that the ??-active isotope of each element is the one having the largest possible decay energy. Rasmussen and others2 suggest that the two or three neutrons just beyond the closed shell of 82 neutrons have decreased binding energies and hence the ??-energy has a maximum about 84 neutrons. Johnson and Nier suggest that the ??-decay of cerium-142 may take place with enough energy to be experimentally observable. Porschen and Riezler3 examined a sample of un-enriched cerium ammonium citrate using nuclear track plates sensitive to ??-particles. No ??-activity was observed after a 30-day exposure of 1.2 mgm. of the cerium salt. In 1957 Riezler and Kauw4 reported an alpha activity for an enriched sample of cerium-142. From their results they calculated a half-life of 5.1 ?? 1015 years with an uncertainty factor of 2. ?? 1959 Nature Publishing Group.