A catalyst-free silicone synthetic process that uses high temperature and pressure water for the hydrolysis and subsequent polycondensation of alkoxysilanes is demonstrated. Polyorganosiloxanes with various structural units including RSiO3/2 (R: methyl and phenyl), Me2SiO2/2, and SiO4/2 were synthesized without organic solvent at temperatures between 200 and 300 degrees C. The driving force of the present catalyst-free reaction seems to be both high Kw of subcritical waters and high reaction temperature. The synthesis depended on the pressure in a reactor because equilibrium is present between the polycondensation of phenyltrimethoxysilane yielding poly(phenylsilsesquioxane) (PPSQ) and depolymerization of PPSQ. The molecular weight of PPSQ decreased as the pressure increases above the pressure threshold ranging between approximately 3 and 7 MPa. It is notable that the molecular weight of PPSQ increased by employing a modified reaction system where non-heated narrow tubing was connected with the heated reactor. Advantageous features of this new process include (1) high product yield within quite shorter process time, (2) easiness of the product isolation, and (3) no contamination of volatile organic compounds in the product due to the organic solvent-free in nature. These may lead to developing an environmentally friendly polyorganosiloxanes manufacturing process. (C) 2007 Elsevier B.V. All rights reserved.