Vertically oriented ordered mesoporous silica membranes have been successfully synthesized in our laboratory in the form of silica plugs filling the macron-sized straight pores of hydrophobic track-etched polycarbonate membrane support. However, these membranes have shown gaps between the plugs and support pore wall which make the membranes unfeasible for use. This paper reports on techniques of synthesis of defect-free ordered mesoporous silica membranes by filling the gaps with microporous silica. Here, the elimination of defects is achieved by filling the membrane gaps with an alkoxysi-lane followed by exposure to humid air to allow controlled hydrolysis and condensation resulting in the formation of microporous silica within the gaps. Molecular probing gas permeation and helium/nitrogen (or oxygen) binary separation tests, coupled with surface characterization methods, show that the final membranes contain ordered mesopores of about 2.7 nm pore diameter, running through the membrane, with gaps sealed by microporous silica having a pore size <0.55 nm.