Abstract: The invention relates to particulate magnesium hydroxide suitable for use as a flame retardant additive for a polymer which has a BET surface area of 13-30 m.sup.2 /g and an average particle size (D.sub.50) of 0.3-1.0 micron.The invention also relates to a process for producing a particulate magnesium hydroxide having a BET surface area of 13-30 m.sup.2 /g and an average particle size (D.sub.50) of 0.3-1.9 micron by calcining a magnesium feedstock to magnesium oxide having a BET surface area of 3-10 m.sup.2 /g; hydrating the magnesium oxide to a magnesium hydroxide hydration product in the absence of added catalyst; and milling the magnesium hydroxide hydration product.The invention further relates to a polymeric article containing particulate magnesium hydroxide according to the invention.