The rate of escape of thoron from solids containing radiothorium depends upon both recoil and diffusion, and is steady at any temperature if the distribution of radioactive material remains unchanged. At very high temperatures the emanating power of ThO 2 (and other indexed oxides) increases with time at constant temperature; the emanating power-temperature characteristics eventually resemble those of a solid with a surface source of thoron. The enhancement of emanating power decays with time in such a way as to prove that Th X is no longer uniformly distributed, although the distribution of the parent Rd Th is unchanged; the Th X atoms, formed by recoil from Rd Th, migrate during their lifetime to trapping sites which, in this case, are on the surface. Similar effects are found with BaThO 3 and BaF 2 , even though Th X (= Ea) can replace Ba isomorphously; these compounds are anionic conductors and have only a low concentration of vacant cation sites at high temperatures. If (e.g. in BaO) vacant sites capable of accepting large cations are present in high concentration, the recoil atoms are trapped and retained in uniform distribution.