A radiochemical analysis of the valence state of uranium isotopes in minerals involves isolation of the U(IV) and U(VI) valence forms and determination of the radioactivity of the 234U and 238U isotopes in each fraction. If the nuclear decay of the uranium in the minerals is accompanied by a redox process, the distribution of the 234U isotope in the U(IV) and U(VI) forms will differ from that of the 238U isotope: oxidation leads to enrichment of the U(VI) fraction with the radiogenic 234U isotope; reduction facilitates enrichment of the U(IV) fraction with 234U. If the mineral is heated to 600–700 °C before the radiochemical analysis, then an equilibrium activity ratio (AR) for both valences. The question arises: What are the processes that bring this about? The authors of this research studied changes of the AR in the U(IV) and U(VI) fractions after isochronous thermal annealing.