Sol-gel reaction of tetraethoxysilane (TEOS) with fumed silica-polyacrylonitrile (PAN) membrane was carried out to prepare hybrid gas permeable membranes for oxygen and nitrogen separation. Various amounts of fumed silica microparticles with a few mum diameters were compounded in PAN-dimethylsulfoxide (DMSO) solution. After casting of the viscous compound solution on a flat sheet with 100 mum thickness, DMSO was evacuated under vacuum at 80degreesC. Then, the silica-PAN composite membranes were treated with TEOS for 1 day at 40degreesC in methanol. Air permeation was examined and compared in silica-PAN composite membranes with and without TEOS treatment. The latter hybrid membranes showed selective oxygen permeability, which depended on amounts of fumed silica in the membrane. The TEOS hybrid PAN membranes have a high ability of oxygen permselectivity for O-2/N-2 gas mixture with alpha(O-2/N-2) = 13-17, when the silica content was in the range of 13-20 wt %. This is attributed to siloxane network formation in hybrid silica-PAN composite membranes. Favorable siloxane network formation resulted in high oxygen permeability of the hybrid composite membranes. (C) 2003 Wiley Periodicals, Inc.