A study of the solubility in the ternary systems NaBH4–NaOH–H2O, KBH4–KOH–H2O, NaOH–NaBO2–H2O и KOH–KBO2–H2O is of special fundamental and practical interest. The first two systems are used as a fuel and as a hydrogen source in hydrogen power engineering that includes the low-temperature fuel cells; borohydrides are converted into metaborates. Hence, the last two systems represent the discharged by-product. The performance of such mixtures is determined by the solubility of their components, negative temperatures being especially critical. Therefore, in the present work the solubility in the ternary systems was studied by means of isothermal saturation at – 10°C. The compositions of the equilibrium liquid and solid phases and compositions of the eutonic and peritonic equilibria, and the coordinates of the homogeneous solution ranges have been determined. The best solubility of the solid components is characteristic of systems NaBH4–NaOH–H2O and KBO2–KOH–H2O.