Hydrolytic polycondensation of methytrialkoxysilanes under the pressure in water and in carbonic acid was investigated. It is shown, that both processes proceed with full conversion of the monomer to form a low molecular soluble polymethylsilsesquioxanes. However, they have a different structure: in water branched compounds were produced and in carboxylic acid polycyclic compounds were synthesized. A type of alkoxy group affects the course of the process. In the case of hydrolytic polycondensation of methyltriethoxysilane under the pressure, unlike methyltrimethoxysilane, full conversion is observed only when additional homogenization of the reaction mixture takes place by vigorous stirring or increasing temperature of the process.