Precipitation of silicas in the emulsion medium from sodium silicate with hexane as the organic phase and hydrochloric or sulphuric(VI) acids as the precipitation agent, has been studied. The effect of the conditions of the reaction of precipitation on the physical and chemical properties of the products has been analysed and the optimum conditions have been established. Synthetic silicas made of spherical particles have been obtained, showing high uniformity for certain contents of the emulsifiers used. By adjusting the amount of the emulsifiers and with hydrochloric acid as the precipitating agent, uniform silicas have been obtained showing a low tendency towards aggregate and agglomerate formation. The physical and chemical properties of the precipitation products obtained have been observed to deteriorate with sulphuric(VI) acid as the precipitating agent, as indicated by particle size distribution curves and SEM images. Agglomeration of the silicas enhances their sedimentation in water, as follows from the sedimentation profiles taken. (C) Koninklijke Brill NV, Leiden, 2010