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The mechanism of the base catalyzed hydrolysis of tetramethoxysilane (TMOS), proposed earlier on the basis of experimental data, is assessed by theoretical methods, i.e.MP2 and B3LYP with 6-31G(d) and Dunning correlation-consistent basis sets. Models considered involve one and two hydrolyzing water molecules attacking (MeO)4Si in the frontside position with a nucleophile (NH3and OH−) in the backside position. This approach allowed us to simulate uniformly the catalytic action of weak and strong bases. It was shown that the presence of a base in the backside position considerably lowers the activation barrier for hydrolysis. The inclusion of the additional water molecule which results in a substantial lowering of the barrier for uncatalyzed hydrolysis does not change noticeably the catalytic effect. In both one- and two-water molecules models the structure of the transition state in the presence of a nucleophile becomes nearly octahedral in which hexacoordinate silicon and four oxygens of the TMOS moiety have a planar arrangement [ABSTRACT FROM AUTHOR]