A process for removing boron contaminant from magnesium halide-containing brine wherein the brine is contacted with active magnesia to precipitate boron-containing magnesium hydrate and to remove at least 50%, and up to at least 95%, by weight of the total boron content of the untreated brine, without reduction of the magnesium halide content of said brine. The active magnesia is employed in an amount of from about 0.1% to about 10% by weight, preferably from about 2% to about 6% by weight, of the untreated brine, which may be heated e.g., from about 140.degree. F to the boiling point thereof, prior to contact with said active magnesia. Subsequently, the treated brine can be used, by reacting it with dolime, to form magnesium hydroxide which, in turn, can be calcined to produce MgO, each of which is a high-purity-low-boron product, i.e., less than 150 ppm boron on an oxide basis, and preferably less than 75 ppm. In addition, the MgO will contain less than 1.5% CaO and can contain, under proper processing conditions, less than 0.8% CaO impurity.