A series of fumed oxides such as silica, titania, alumina, silica/alumina (SA), silica/titania (ST), and alumina/silica/titania (AST), initial and hydrothermally treated (HTT) in the steam phase at T-HTT = 150, 250, and 350degreesC was studied by adsorption, AFM, XRD, FTIR, and theoretical methods. Diminution of the size of primary particles (corresponding to increasing S-BET) of initial;silica and mixed oxides results in enhancement of their structural changes on HTT with elevating T-HTT and increasing density of packing of primary particles in the secondary structures. Relative changes in the texture of treated fumed silicas are smaller than those of mesoporous silica gels occurring under similar HTT conditions. On HTT, aggregates of primary particles and their agglomerates become denser but their surface layers become looser because of transfer of silica fragments from one particle to another, and the smaller the initial primary particles, the greater the relative diminution of the specific surface area SBET for the same type of primary particle packing in aggregates. Relative changes in the pore volume V-p (or V-BJHd) on HTT are more complex than that of SBET, as for many samples the Vp value increases especially at T-HTT = 150 degreesC. Alumina and titania partially inhibit structural changes on HTT, which decrease in the series silica > SA > AST approximate to ST. (C) 2003 Elsevier Inc. All rights reserved.