A low-temperature (300 degrees C) CVI technique was employed to modify commercial sol-gel silica membranes, in order to close meso- or micropores and repair defects often formed in their top sol-gel layer. Membranes were characterized before and after modification in terms of permeance and permselectivity with the use of He, H-2, N-2, CO2 and SF6 gas probe molecules. The sol-gel silica membranes had a low selectivity indicative of a Knudsen diffusion mechanism which reveals the presence of defects on the silica layer. The pore size of the CVI modified silica membranes decreased to less than 0.55 nm with a concomitant decrease of permeance and a significant improvement of permselectivities (H-2/N-2: 91, H-2/CO2: 50, He/N-2: 244 at 300 degrees C). Activation energies of permeation for various gases reveal a significant improvement of permselectivity with temperature for industrially important gas pairs, such as H-2 and He over CO2 and N-2. This hybrid CVI technique on sol-gel silica membranes provides an effective method for closure of pores and for the repair of top-layer defects, at temperatures (300 degrees C) where available sealing technologies can be used. (c) 2009 Elsevier B.V. All rights reserved.