Nanoporous silica composite membranes for gas separation have been synthesized by dip-coating the tetrapropylammonium (TPA)-templating silica sols on tubular alumina supports (pore size 2.8-100 nm), followed by eliminating the template via heat-treating at 550-600degreesC. The NMR spectroscopy of TPABr-silica hybrid composites obtained from the templated silica sols confirmed that TPA molecules (i.e., final pores) were uniformly distributed in the silica matrix. The average pore size and the specific surface area of an unsupported membrane prepared by firing the TPABr (6 wt%)-silica hybrid composite at 600degreesC were below 18 Angstrom and 830 m(2)/g, respectively. Any defects such as cracks or pin-holes on the surface of amorphous silica composite membranes were not observed. The CO2/N-2 separation factor of their composite membranes varied from 3.2 to 10.3 and their gas permeability from 10(-8) to 10(-9) mol/m(2) . s . Pa depending on the microstructure of aluminar supports.