PDMS (polydimethylsiloxane)/TEOS (tetraethoxysilane) system Ormosils (organically modified silicates) can be rubbery or rigid, depending on their chemical composition and processing conditions. In order to determine the relationship between the viscosity and the structure of the Ormosil solutions through the sol-to-gel transition, five kinds of sols (three levels of PDMS, three levels of H2O) were prepared with refluxing. Viscosities were measured using an oscillation method with a cone-and-plate type apparatus. Liquid state 29Si-NMR analyses were made to obtain structural information of the solution. The composition of the starting solution affected the sol-to-gel transition process. The degree of hydrolysis, depending upon the H2O content, controlled the reaction rate and structure of the polymer. Addition of PDMS also contributed to the acceleration of the sol-to-gel transition due to larger PDMS copolymerizing units.