Preparation of organically modified silicon dioxides

In a process for the production of a porous silicon dioxide bearing organosilyl groups on the surface thereof by the hydrolytic polycondensation of a tetraalkoxysilane or a polyalkoxysiloxane in the heterogeneous phase, the improvement which comprises conducting the hydrolytic polycondensation in the presence of a monomeric organoalkoxysilane, the organogroup of which is not hydrolyzable under conditions of said hydrolytic polycondensation, thereby producing a porous silicon dioxide bearing organic groups which are part both of the skeleton structure and the surface of the porous silicon dioxide.