Organically modified silicates (ormosils) of high hardness were prepared by the reaction of TEOS (tetraethoxysilane) and PDMS (polydimethylsiloxane) aided by ultrasonic irradiation, which was chosen as the method for concentration of the solutions. The mechanisms leading to the hard ormosil formation were examined by liquid state 29Si NMR spectroscopy. PDMS chains were found to be broken into shorter chains and/or cyclic D4 tetramers during the reaction and finally, all the PDMS chains were chemically incorporated as short chains into silica networks. Structural models of the mechanisms are also proposed. Elastic moduli and Vickers hardnesses of the hard ormosils were measured. Vickers hardnesses of the hard ormosils were compared with those of some glasses and the hardest transparent plastics and the hard ormosils were much harder than the plastics and a little softer than soft glasses. Theoretical models have been developed for the calculations of the elastic moduli and Vickers hardnesses and agreed well with the experimental results. Predictions based on these models indicate that even higher elastic moduli and Vickers hardnesses of the hard ormosils can be obtained when Al2O3, ZrO2 and TiO2 are substituted for SiO2. 30 mol% TiO2-containing ormosils of high hardness were also prepared aided by ultrasonic irradiation. TEOS and tetraisopropyltitanate (TIPT) were used as the inorganic components. Dimethyldiethoxysilane (DMDES) was used instead of PDMS as the organic precursor. The reactions among alkoxides were examined by liquid state 29Si NMR spectroscopy. While the reactivity between TIPT and PDMS was much lower than the reactivity between TIPT and hydrolyzed TEOS, the reactivity between TIPT and hydrolyzed DMDES was not so different from the reactivity between TIPT and hydrolyzed TEOS, and thus DMDES was used as the organic precursor instead of PDMS. Elastic moduli and Vickers hardnesses of the hard ormosils of the TEOS/TIPT/DMDES system were measured and even higher elastic moduli and Vickers hardnesses than those of the TEOS/PDMS system hard ormosils were obtained. Also, the calculated results from the theoretical models agreed well with the experimental results.