Process improvements in silica membrane fabrication, especially the use of clean-room techniques, resulted in silica membranes without detectable mesoscopic defects, resulting in significantly improved transport properties. Supported membranes calcined at 400 degrees C were 30 nanometers in thickness, showed a H-2 permeance at 200 degrees C of 2 x 10(-6) moles per square meter per second per Pascal (mol m(-2) s(-1) Pa-1), and had a CH4 permeance more than 500 times smaller. Molecules larger than CH4 were completely blocked. Silica membranes calcined at 600 degrees C showed no detectable CH4 flux, with a H-2 permeance of 5 x 10(-7) (mol m(-2) s(-1) Pa-1) at 200 degrees C. These results signify an important step toward the industrial application of these membranes such as purification of H-2 and natural gas as well as the selective removal of CO2.