Sodium antimonate particles wherein 40% or more of the particles are less than 1 micron in diameter and the range of particle diameters is between 0.1 to 8 microns impart an enhanced degree of flame retardancy to synthetic organic polymer compositions containing a halogen source. The flame retardancy is further improved if any alkaline impurities are removed by treating the particles with acidic reagents as required until the aqueous phase of a slurry prepared using distilled water and containing 10% by weight of the particles exhibits a pH of between 6.8 and 7.