Molecular sieving silica-based membranes were prepared on porous polymeric supports by atmospheric-pressure plasma-enhanced chemical vapor deposition. The effect of polymeric support pore size on the gas permeation properties of polymer-supported silica-based membranes was investigated using nanofiltration (NF) and ultrafiltration membranes as polymeric supports. The silica-based layer was uniformly formed on the surfaces of the polymeric supports, irrespective of their different pore sizes. The selectivity of the resultant polymer-supported membranes was found to increased with decreasing support pore size. The NF-supported membrane exhibited an improved H2/N2 permeance ratio of > 40, which was comparable to those of ceramic-supported membranes, demonstrating the successful synthesis of molecular sieving silica-based membranes onto a porous polymeric support.