Abstract: A magnesium hydroxide of low lime and boron content is produced from a brine containing magnesium chloride and over 1 ppm boron (on a B.sub.2 O.sub.3 basis) by treating it to reduce its CO.sub.2 content to less than 15 ppm and reacting it with an excess of active lime in a first reaction stage, decanting the spent brine from the precipitated high lime magnesium hydroxide, and then reacting the high lime magnesium hydroxide with additional brine in a second reaction stage to produce a magnesium hydroxide containing less than 1% CaO and less than 0.1% B.sub.2 O.sub.3 on the ignited basis. A portion of the high lime magnesium hydroxide from the first reaction stage is recycled to that stage as seed. Before being discarded, the spent brine from the first reaction stage may be reacted with treated brine to precipitate magnesium hydroxide of relatively high CaO and B.sub.2 O.sub.3 content.