Obtaining wide porous silicas by means of hydrothermal treatment (HT) of mesoporous silicas is the most reliable method from the point of view of the reproducibility of obtained data. Silica gels with relatively narrow pore distributions by size could be obtained during the first hours of HT. Increase of the duration and rise of the temperature of HT leads to uncontrolled change of the silica gel structure. Existing theories explaining the mechanism of the change of porous characteristics during hydrothermal treatment do not completely describe the reasons for one or another change. A new approach explaining the mechanism of the change in porous characteristics of the silica during HT, based on the assumption that the water in the pores is in "motion state," has been suggested in the present work. Theories did not consider water state in the pores during HT. An attempt to explain the changes of the porous structure of the silica gels during HT is undertaken in the present manuscript. (C) 2004 Elsevier Inc. All rights reserved.