A potentiometric titration technique utilising a hydrogen-electrode concentration cell was applied to study the hydrolytic behaviour of the magnesium(II) ion in 0.10 and 1.0 mol kg^–1 sodium chloride at 60, 100, 150 and 200 °C. Data analysis indicates the presence of Mg²⁺(aq) and the solid species Mg(OH)₂(s), only. Independent measurements have confirmed that [Mg(OH)]⁺(aq) is a negligible ionic species under the prevailing experimental conditions. Experimental evidence indicated that the solid species formed both rapidly and reversibly. The equilibrium quotients for brucite formation were determined for each temperature and ionic strength investigated and the corresponding enthalpy and entropy of formation calculated. Formation constants have also been calculated at zero ionic strength using the Debye–Hückel equation, and these data, together with some other literature data, comply with the equation log β_12s⁰= 2.49 –(5847/T) where log β_12s⁰ is the logarithm of the formation constant of brucite at zero ionic strength and T is the temperature in Kelvin.