Polyimide (PI)-silica composite membranes and their derived carbon-silica composite membranes were prepared for gas separation. The polyimide-silica composite membranes were prepared using the sol-gel technique, in which the polyimide matrix was synthesized by the condensation of pyromellitic dianhydride (PMDA) and 4,4'-oxydianiline (ODA) while the inorganic phase was prepared by the in situ hydrolysis of tetraethyl orthosilica (TEOS) and a silane coupling agent, (3-aminopropyl)triehtoxysilane (APTES). The derived carbon-silica composite membranes were prepared by the pyrolysis of the polyimide-silica composite membranes at 900 degrees C under vacuum. The structures of the membranes were characterized by scanning electron microscopy (SEM), energy dispersive X-ray (EDX) spectroscopy, and X-ray diffraction (XRD) technique. The gas (He, CO2, N-2 and O-2) permeabilities of the polyimide-silica composite membranes and carbon-silica composite membranes were investigated. With the introduction of the silica, there was no significant enhancement of the gas separation in the resulting polyimide-silica composite membranes over the polyimide membrane, which still suffered a "trade-off" relationship between permeability and selectivity. However, the derived carbon-silica composite membranes exhibited better gas separation properties. The C-SiO2 28% composite membrane produced the highest permeances of 1042.18, 991.21, 296.03 and 155.26 GPU for He, CO2, O-2 and N-2, respectively, which were 21.61, 137.67, 103.15 and 150.74 times, respectively, of those of the pure carbon membrane. The C-SiO2 11% composite membrane produced the highest selectivities of 37.57, 36.61 and 7.09 for He/N-2, CO2/N-2 and O-2/N-2, respectively, which had surpassed the Robeson's upper bound for these gas pairs. The agreement between the pure gas selectivity and the mixed gas selectivity for O-2/N-2 gas pair was reasonable, ranging from -8.30% to +16.61% using the pure gas selectivity as the base except for C-SiO2 23% composite membrane which showed +35.81% difference. (C) 2013 Elsevier B.V. All rights reserved.