Аннотация:Theoretical reasoning shows that measurement of the magnetic susceptibility makes it possible to carry out a high-sensitivity analysis of seven-valent neptunium compounds for neptunium(VI) content. This communication is devoted to the study of the purity and stability of neptunium(VII) compounds whose composition corresponds to the formula MNpO4.nH2O, where M = Li, Na, K and Cs. The experimental data show that the cesium compound is free of neptunium(VI). In addition to this compound, the potassium salt is also stable over time. Sodium and lithium neptunates(VII) are somewhat less stable during storage. After two and a half months, 2-3% of the neptunium(VII) was reduced down to the six-valent state. On the whole, with an increase in the radius of the alkali metal cation in the compound MNpO4.nH2O, their stability increases. Thus the magnetic susceptibility measurement method can be convenient for monitoring the purity and stability during storage of seven-valent neptunium compounds.