New aminosilanes with hydrolysable Si-N bond(s) were prepared by the amination of chlorosilanes and used as hydrophobic modifiers of SiO2 surface. The aminosilanes are rather stable towards hydrolysis in air compared to their chlorosilane analogs. The modified SiO2 was applied as a gate insulator of an organic thin-film transistor with a vapor-deposited film of pentacene as the active material. The transistor with the aminosilane-modified SiO2 showed two- to threefold higher hole mobility than the device with bare SiO2. [GRAPHICS] .