Incomplete interaction of silicon tetrafluoride, SiF4, with water2SiF4 + HOH → Si2F6O (SiF3OH) + 2HF (1)is one of most important and still not clearly understood chemical reaction of this substance. This work is devoted to the study of elementary steps of reaction (1) in the gas phase at moderate temperatures using quantum-chemical methods and high-resolution FTIR spectroscopy in gas phase and low-temperature Ar matrix.The energies and thermodynamic parameters of elementary reactions involved in (1) were studied by ab initio quantum chemical methods (MP4//MP2/6-311G(2d,2p)) and the density functional theory (B3LYP/6-311G(2d,2p)). It was shown that he initial stages of hydrolysis may be represented by only two reactions: formation of trifluorohydroxysilane SiF3OH and hexafluorodisiloxane Si2F6O which are the most thermodynamically favorable. For these elementary reactions, the transition states were located and the kinetic parameters were estimated.The theoretic results were verified by the FTIR spectroscopy of SiF4 and H2O mixtures registered in gas phase, and by low-temperature FTIR spectroscopy in Ar matrix. Formation of SiF3OH and Si2F6O was confirmed in both cases. In Ar matrix the bands assigned to (SiF4)2 dimer and SiF4H2O complex were observed.Thermodynamics of hydrolysis reaction was studied in gas phase (FTIR method). The experimentally obtained Kp values 12.4105 and 2.5105 are in a good agreement with the theoretic results.