This paper reports on a new microporous composite silica membrane prepared via acid-catalyzed polymeric route of sol-gel method with tetraethylorthosilicate (TEOS) and a bridged silsesquioxane [1, 2-bis(triethoxysilyl)ethane, BTESE] as precursors. I. stable nano-sized composite silica sol with a mean volume size of similar to 5 nm was synthesized. A 150 nm-thick defect-free composite silica membrane was deposited on disk support consisting of macroporous alpha-Al2O3 and mesoporous gamma-Al2O3 intermediate layer by using dip-coating approach, followed by calcination under pure nitrogen atmosphere. The composite silica membranes exhibit molecular sieve properties for small gases like H-2, CO2, O-2, N-2, CH4 and SF6 with hydrogen permeances in the range of (1-4)x10(-7) mol.m(-2).s(-1).Pa-1 (measured at 200 degrees C, 3.0x10(5) Pa). With respect to the membrane calcined at 500 degrees C, it is found that the permselectivities of H-2 (0.289 nm) with respect o N-2 (0.365 nm), CH4 (0.384 nm) and SF6 (0.55 nm) are 22.9, 42 and >1000, respectively, which are all much higher than the corresponding Knudsen values (H-2/N-2 = 3.7, H-2/CH4 = 2.8, and H-2/SF6 = 8.5).