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 cells; borohydrides convert into metaborates. The performance of fuel mixtures is determined by their solubility in water. A general analysis of the solubility in the ternary systems NaBH4 + NaOH + H2O and KBH4 + KOH + H2O within the range (-10 to 50) degrees C is done, and regularities and features of the temperature-induced transformation of the phase diagrams of the systems are revealed. The systems with sodium and potassium ions are 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 borohydrides and hydroxides is characteristic for both systems. The composition of crystalline hydrates depends on the component ratio in the mixture and the cation nature.