We demonstrate a "template" approach to prepare microporous inorganic membranes exhibiting high flux combined with hi,oh selectivity, overcoming limitations inherent to both conventional inorganic (sol-gel, CVD) and organic membrane approaches. Hybrid organic-inorganic polymers prepared by co-polymerization of tetraethoxysilane (TEOS) and methyltriethoxysilane (MTES) were deposited on commercial asymmetric alumina supports. Heat treatments were employed to densify the inorganic matrix and pyrolyze the methyl ligands, creating a continuous network of micropores. Resulting membranes exhibited very high CO2 permeance values (2.57 X 10(-3) cm(3)/cm(2) s cm Hg) combined with moderate CO2/CH4, selectivities (12.2). Subsequent derivatization of the pore surfaces with monomeric TEOS significantly increased CO2/CH4 separation factors (71.5) with only a modest reduction in CO2 permeance (2.04 X 10(-4) cm(3)/cm(2) s cm Hg). Combined CO2/CH4 selectivity factors and CO2 fluxes exceeded those of all known organic membranes.