The process of silicas formation in emulsion system in the reaction of precipitation from water solutions of sodium silicate and hydrochloric acid has been studied. The effects of silica surface modifications with silane coupling agents from the group of alkoxysilanes with different functional groups have been determined. Both unmodified and modified silicas have been thoroughly characterised by scanning electron microscopy (SEM) and dynamic light scattering (DLS) and other methods. The effect of alkoxysilanes on the diameter distribution of the modified silica is insignificant; the presence of silane coupling agents causes a small increase in the particle diameter and a small increase in the tendency towards agglomeration, moreover the particles of silicon dioxide precipitated are characterised by spherical shape. The course of the electrokinetic curves depends mainly on the proton affinity of the modifiers. Significant changes in the stability of silica dispersions have been found as a result of modification with N-2-(aminoethyl)-3-aminopropyltrimethoxysilane. This compound causes the greatest increase in the tendency towards agglomeration of the primary particles. The modification mechanism has been established to be chemisorption, as indicated by the C-13 and Si-29 CP MAS NMR spectra. The products have been shown to have typical mesoporous structure. The surface area of the unmodified silica is 133 m(2)/g, while those for the modified silicas vary from 115 to 182 m(2)/g. (C) 2009 Elsevier B.V. All rights reserved.