Nanoporous silica composite membranes for gas separation have been synthesized by the organic-templating sol-gel technique and their CO2/N-2 separation efficiency has been investigated. Stable organic-templating silica sots capable of coating nanoporous amorphous silica layers on SiO2/alpha-Al2O3 composite supports were prepared by the two-step acid-catalytic process using the mixture of TEOS and template such as MTES (methyltriethoxysilane) being covalently bonded with TEOS or TPABr (tetrapropylammonium bromide) non-reacted. The SiO2/alpha-Al2O3 composite supports (pore size 1.6 nm) prepared by pressurized coating of the porous alpha-alumina tube with a nanoparticulate silica sol were found to be more suitable than pure alpha-Al2O3 composite supports for synthesizing crack-free membranes which have the high CO2/N-2 separation efficiency. The N-2 permeability of organic-templated silica composite membranes synthesized was about 10(-8) mol/m(2) . s . Pa and the CO2/N-2 separation factor of the MTES-templated silica composite membranes was enhanced up to 19 at room temperature by surface-modifying the templated silica membrane with the TEOS-EtOH solution.