Concentrated water-alkaline solutions of sodium and potassium borohydrides are used as a fuel and as a hydrogen source in hydrogen power engineering, including low-temperature fuel cells, where borohydrides are converted into metaborates. The performance of such mixtures is determined by the solubility of their components, negative temperatures being especially critical. The solubility in the ternary systems NaBH4 + NaOH + H2O, KBH4 + KOH + H2O, NaBO2 + NaOH + H2O, and KBO2 + KOH + H2O was studied by means of isothermal saturation at -10 degrees C. The compositions of the equilibrium solid phases and of mixtures corresponding to nonvariant equilibrium points were determined; the solubility diagrams of the systems were plotted. The presence of a range of homogeneous solutions and fields of crystallization of ice, anhydrous, and hydrated forms of the salts and hydroxides is characteristic of all four systems at the said temperature. The composition of crystalline hydrates depends on the ratio of the components in the mixture and on the cation nature. Of the borohydride systems, the best solubility was observed in that with NaBH4, whereas in the metaborate systems it was that with KBO2.