An ambient temperature C-ALD (with NH3 as catalyst) method was employed to modify sol-gel silica membranes, in order to reduce their pore size and upgrade their gas separation performance. Membranes were characterized before and after modification in terms of permeance and permselectivity with the use of He and N2, as gas separation indicators. The application of the proposed method significantly improved the permselectivity of the membranes accompanied with a minor permeance reduction. The process temperature renders it ideal for application in commercial membrane modules which utilize economic, current available, sealing technologies. The performance of the C-ALD modified silica membranes in various gas separation applications was further examined, indicating that they are potential candidates for H2 separation in various chemical processes. Finally, the stability of the C-ALD modified silica membranes in H2O was studied, indicating some performance loss during the first hours of the membrane's exposure to a H2O atmosphere, which nevertheless remained high and steady then on.