Silica-polyimide microcomposite membranes were prepared on gamma-alumina-coated alpha-alumina support tubes, and their gas permeation properties were evaluated with He, N-2 and CO2. Smoothing of the substrate surface and hybridization of silica and polyamic acid were both effective to form defect-free thin composite membranes. The CO2 permeance of a membrane with a silica content of 68 wt% was one order of magnitude higher than that of a polyimide membrane having the same thickness. The permselectivity of CO2 to N-2 was 30 at 30 degrees C and 13 at 100 degrees C. Contributions of the silica and polyimide phases to permeance of the composite membrane were analyzed with a two-phase permeation model. The effective thickness of the rate-controlling polyimide phase was less than one-tenth of the total thickness of the silica-polyimide membrane.