A SiOC membrane with high oxidative stability for gas separation was tailored by utilizing vinyltrimethoxysilane, triethoxysilane, and 1,1,3,3-tetramethyldisiloxane as Si precursors. Amorphous SiOC networks were formed via the condensation of Si–OH groups, the hydrosilylation of Si–H and Si–CH=CH 2 groups, and a crosslinking reaction of Si–CH 3 groups, respectively. The crosslinking of Si–CH 3 groups at temperatures ranging from 600 to 700 °C under a N 2 atmosphere was quite effective in constructing a Si–CH 2 –Si unit without the formation of mesopores, which was confirmed by the results of N 2 adsorption and by the gas permeation properties. The network pore size of the SiOC membrane calcined at 700 °C under N 2 showed high oxidative stability at 500 °C and was appropriate for the separation of large molecules (H 2 /CF 4 selectivity: 640, H 2 /SF 6 : 2900, N 2 /CF 4 : 98). A SiOC membrane calcined at 800 °C showed H 2 /N 2 selectivity of 62, which was approximately 10 times higher than that calcined at 700 °C because the SiOC networks were densified by the cleavage and redistribution reactions of Si–C and Si–O groups.