This paper presents the investigations on the mechanism of high pressure hydrothermal treatment (HTT) of pyrogenic silicas as well as titania- and alumina-aerosils. The aerosils were modified by using different sizes of primary particles (A-175, A-300 and A-380) as well as by containing 8% and 9% w/w of the Al 2O 3 and TiO 2 respectively. The HTT process was performed in the gas phase (overheated steam with and without addition of ammonia) in the temperature range 100–350°C. The changes of surface properties of the adsorbents tested caused by HTT were determined on the basis of adsorption isotherms of water, methanol, carbon tetrachloride, n-hexane and benzene, and using the data relating to chemisorption of hexamethyldisilazane as well as on the basis of specific surface area determined by the method of thermal desorption of argon. It was stated that the hydrothermal treatment of aerosil leads to the increase of hydroxylation degree of their surface. In the HTT mechanism the processes of mass transfer of silica in the gas (steam) phase play a very important role. The intensity of these processes depends on the size of primary particles (globules) of modified adsorbent, on its chemical composition, and on the presence of ammonia in steam. The possibilities of utilizing the processes occurring in the vapour (steam) phase to prepare porous coverages (layers) were demonstrated.