Thermally stable organosilica amorphous membranes were fabricated by in situ hydrosilylation of vinyl and hydrosilyl groups in Si precursors for the formation of a Si–C–C–Si unit, as well as the Si–O–Si unit via the conventional hydrolysis and condensation of silanol groups. Two types of SiOC membranes (VTT-type prepared by vinyltrimethoxysilane (VTMS), triethoxysilane (TRIES) and tetramethyldisiloxane (TMDSO); and VT-type prepared by VTMS and TRIES) that consisted of a Si–C–C–Si unit created by the hydrosilylation of vinyl and hydrosilyl groups in a Si precursor were fabricated at temperatures higher than 300 °C under N2. Single-gas permeation properties for SiOC membranes at temperatures ranging from 100–500 °C were examined to determine the effect of a silica precursor on the size of an amorphous network. A SiOC membrane (VTT-type) fabricated at 400 °C under N2 showed a H2 permeance of 5.3 × 10−7 mol m−2 s−1 Pa−1 with H2/CO2, H2/N2, H2/CH4 and H2/CF4 permeance ratios of 5.6, 15, 18 and 485, respectively, at 400 °C. The permeance of H2 for a VTT-type membrane heat-treated at 550 °C in air significantly increased from 5.3 × 10−7 to 2 × 10−6 mol m−2 s−1 Pa−1 with a decrease in the H2/CF4 permeance ratio from 485 to 90 due to the combustion of the CH3 groups. The SiOC membrane (VT-type) showed high thermal stability in air at 550 °C with a H2 permeance of 3.0 × 10−7 mol m−2 s−1 Pa−1 and H2/CH4 and H2/CF4 permeance ratios of 50 and 400, respectively, at 400 °C.