[en] The possibility of applying the method of sublimation of chlorides in the separation of neptunium from transplutonium and fission elements for subsequent radiometric or γ-spectroscopic determination of ²³⁹Np and, correspondingly, parent isotope ²⁴³Am was investigated. The method made it possible to separate very small and in particular indicator amounts of radioactive isotopes in a radiochemically pure state. The behaviour of ²³⁹Np was studied in relation to a number of factors: the nature of the supporting material, the temperature and duration of the chlorination-sublimation process, the composition of the sample analyzed, the presence of macrocomponents and others. A marked rate of sublimation of ²³⁹Np on Ar+SOCl₂ flow was observed at temperatures higher than 550 and 600 deg C with quartz and praphite supporters respectively. With an increase in temperature the rate of sublimation increased. More than 98% of ²³⁹Np was sublimated within 50 minutes with quartz supporter at 640-650 deg C but only 20% of ²³⁹Np was sublimated with graphite supporter under the same conditions. The experiments were therefore carried out with the use of quartz supporter. The rate of the chlorination sublimation process was the same for solutions of ²³⁹Np in hydrochloric, nitric and thiocyanic acid at different concentrations. The solutions of ²³⁹Np in sulfuric and phosphoric acid showed a sharp decrease in the rate of the chlorination-sublimation process. The degree of ²³⁹Np purification from americium, curium and rare-earth fission elements was about 10⁴. This made it possible to carry out radiometric determinations of ²⁴³Am from the amount of ²³⁹Np present in equilibrium with the parent isotope in samples that were free of other fission products after separation of ²³⁹Np through a single process of sublimation of the chlorides at 640-650 deg C for a period of 60-70 minutes