Many organically modified silicates (ormosils) can be prepared by the sol-gel method with very different mechanical properties by varying the ratio of polydimethylsiloxane (PDMS) to tetraethoxysilane (TEOS) contents and processing conditions. With a low PDMS concentration, the ormosils are harder, stiffer and stronger than those with higher concentration of PDMS. Even harder ormosils are possible when ultrasonic irradiation is used during synthesis. As the PDMS concentration is increased, the ormosils take on a more flexible nature, and over a critical concentration actually become rubbery. These new rubbery materials can contain as much as 75% inorganic components, and have more stable mechanical properties than commercial rubbers at elevated temperatures. Effects of PDMS addition to the network structure and mechanical properties of the ormosils were investigated.