Fumed silicas unmodified (S-BET = 378 m(2)/g) and modified (379-285 m2/g) by hexamethyldisilazane reacting with silanols (concentration of grafted trimethylsilyl groups CTMS = 0.09-0.79 mmol/g) were studied by means of the NMR, IR, differential thermogravimetry, atomic force microscopy, microcalorimetry, adsorption, and theoretical methods. Variation in the surface composition and changes in the structural characteristics of primary and secondary particles lead to the nonlinear dependence of the Gibbs free energy of interfacial water, the heat of immersion of silicas in water, and the chemical shift delta(H)(T) of water adsorbed from air on C-TMS, despite a nearly linear decrease in water adsorption (at p/p(0) approximate to 0.8 for 24- 72 h) with CTMS. An increase in CTMS causes alterations in the structure of the hydrogen bond network in the interfacial water and other properties of water strongly (700-300 mg/g silica) and weakly (1400-500 mg/g) bound to the silica surfaces.