The synthesis and properties of hydrophobic silica membranes are described. These membranes show very high gas permeance for small molecules, such as H-2, CO2, N-2, O-2, and CH4, and permselectivities of 20-50 for these gases with respect to SF6 and larger alkanes like C3H8 and i-C4H10. The membranes are prepared by repeated dip coating of supported gamma-alumina membranes in a silica sol solution, followed by drying and calcining. The hydrophobic nature of the membranes is obtained by adding methyl-tri-ethoxy-silane (MTES) to the sol prepared by acid-catalysed hydrolysis and condensation of tetra-ethyl-ortho-silicate (TEOS). The double silica membrane layer has a total thickness of 60 nm and a pore empty set of ca. 0.7 nm. The membranes are 10x more hydrophobic than the state-of-the-art silica membranes which makes them more suitable for application in humid process streams. Besides that, the very high permeance obtained for N-2 and O-2 Of 4 and 7 x 10(-7) mol/m(2) s Pa, respectively, offer perspectives on dedicated air purification in which larger impurity molecules are blocked by molecular sieving effects.