Concentrated water-alkaline solutions of sodium and potassium borohydrides are used as a fuel and hydrogen source in hydrogen power engineering, including low-temperature fuel.borohydrides Convert into metaborates. The performance of fuel mixtures is determined by their solubility in water. The solubty in the ternary systems NaBO2 + NaOH + H2O and KBO2 + KOH + H2O was studied by means of isothermal Saturation within the range (-10. to 50)degrees C. The composition of equilibrium solid phases and those of mixtures corresponding to nonvariant equilibrium points :were determined; the solubility diagrams of these systems were constructed. The systems with sodium and potassium ions are considerably different in relation to the temperature influence on the solubility Of the solid components. The presence of a range of, homogeneous solutions and fields of crystallization of hydrated and anhydrous forms, of the metaborates and hydroxides is characteristic for both Systems at all investigated temperatures. The composition of crystalline hydrates depends on the component ratio in the:mixture and the cation nature.