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In this work, solubility measurements regarding boracite [Mg₃B₇O₁₃Cl(cr)] and aksaite [MgB₆O₇(OH)₆·2H₂O(cr)] from the direction of supersaturation were conducted at 22.5 ± 0.5 °C. The equilibrium constant (log₁₀K⁰) for boracite in terms of the following reaction, Mg₃B₇O₁₃Cl(cr) + 15H₂O(l) ⇌ 3Mg²⁺ + 7B(OH)₄^– + Cl^– + 2H⁺ is determined as -29.49 ± 0.39 (2σ) in this study. The equilibrium constant for aksaite according to the following reaction, MgB₆O₇(OH)₆•2H₂O(cr) + 9H₂O(l) ⇌ Mg²⁺ + 6B(OH)₄^– + 4H⁺ is determined as -44.41 ± 0.41 (2σ) in this work. This work recommends a set of thermodynamic properties for aksaite at 25 °C and 1 bar as follows: ΔH$0\atop{f}$ =-6063.70 ± 4.85 kJ·mol^-1, ΔG =-5492.55 ± 2.32 kJ·mol^-1, and S⁰ = 344.62 ± 1.85 J·mol^-1·K^-1. Among them, ΔG$0\atop{f}$ is derived from the equilibrium constant for aksaite determined by this study; ΔH$0\atop{f}$ is from the literature, determined by calorimetry; and S⁰ is computed in the present work from ΔG$0\atop{f}$ and ΔH$0\atop{f}$. This investigation also recommends a set of thermodynamic properties for boracite at 25 °C and 1 bar as follows: ΔH$0\atop{f}$ =-6575.02 ± 2.25 kJ·mol^-1, ΔG$0\atop{f}$ =-6178.35 ± 2.25 kJ·mol^-1, and S⁰ = 253.6 ± 0.5 J·mol^-1·K^-1. Among them, ΔG$0\atop{f}$ is derived from the equilibrium constant for boracite determined by this study; S⁰ is from the literature, determined by calorimetry; and ΔH$0\atop{f}$ is computed in this work from ΔG$0\atop{f}$ and S⁰. The thermodynamic properties determined in this study can find applications in many fields. For instance, in the field of material science, boracite has many useful properties including ferroelectric and ferroelastic properties. The equilibrium constant of boracite determined in this work will provide guidance for economic synthesis of boracite in an aqueous medium. Similarly, in the field of nuclear waste management, iodide boracite [Mg₃B₇O₁₃I(cr)] is proposed as a waste form for radioactive ¹²⁹I. Therefore, the solubility constant for chloride boracite [Mg₃B₇O₁₃Cl(cr)] will provide the guidance for the performance of iodide boracite in geological repositories. Boracite/aksaite themselves in geological repositories in salt formations may be solubility-controlling phase(s) for borate. Finally, solubility constants of boracite and aksaite will enable researchers to predict borate concentrations in equilibrium with boracite/aksaite in salt formations.