An understanding of brucite stability at 25 degrees C. and 1 atm constitutes a starting point for hydrolysis studies in the MgO-SiO (sub 21-CO 2 -H 2 O system over a range of temperature and pressure. Lack of agreement among published work prompted the author to investigate brucite stability both by solubility and by precipitation techniques. The association of magnesium and hydroxyl ions to form the complex ion, MgOH (super +) , assumed a modest importance during these investigations. Thermodynamic dissociation constants at STP for the reactions: Mg(OH) 2c = Mg aq (super ++) + 2OH (super -) aq Mg(OH) (super +) aq = Mg aq (super ++) + OH (super -) aq were experimentally determined to be 10 (super -11.15+ or -0.03) and 10 (super -2.60+ or -0.25) , respectively. The effect of particle size on solubility is reported here for brucite. Particle sizes were measured from electron-micrographs taken at various stages during the aging of precipitated magnesium hydroxide. Solubility does not decrease to a sensible extent when the diameter of the (0001) faces of the brucite grains exceeds a value of 1.5 X10 (super -5) cm. Published P-T curves for the reaction, Mg(OH) 2c = MgO c + H 2 O g , are evaluated with the help of data obtained from the grain size-solubility experiment.