Influence of mechanochemical activation on porous structure and surface properties of porous and nonporous silicas in air, water and ethanol has been studied. Milled samples have been investigated with help of nitrogen adsorption-desorption, mercury porometry, thermogravimetric analysis, FTIR and ESR spectroscopy. It has been revealed that destruction of porous structure of silicagel at dry milling and, on the contrary, formation of porous silica from non-porous powders (aerosils) during treatment in liquid media occurs. (C) 2010 Elsevier B.V. All rights reserved.