Nanoporous carbon/silica (C/SiO2) membranes (see Figure) with high permeabilities and high gas selectivities are prepared by the controlled pyrolysis of polyimide/silica (PI/SiO2) composites obtained from polymerization of alkoxysilanes in situ via a sol-gel reaction. The silica content and the nature of the silica network incorporated in the carbon matrix can be used to tune the gas permeation and separation properties of the final C/SiO2 membranes.