A novel membrane is described consisting of an ultrathin 20-30 nm permselective layer of silica deposited on an intermediate multilayer gamma-alumina substrate with a graded structure. The alumina substrate is formed on top of a macroporous alumina support by sequentially dipping and calcining a series of dilute solutions containing boehmite (AlOOH) sots of different particle sizes. The size of the sol particles is tuned by precisely controlling synthesis parameters including acid type, acid concentration and hydrolysis time. The topmost silica layer is deposited on top of the intermediate alumina layer by chemical vapor deposition of a silica precursor, tetraethylorthosilicate, in an inert atmosphere. Cross-sectional images of the membranes obtained from scanning electron microscopy (SEM) show that the intermediate alumina layers are formed from particles of progressively smaller size so as to form a smooth interface between the rough support and the topmost amorphous silica layer. The resulting silica-on-alumina composite membrane had high permeance of 5.0 x 10(-7) mol m(-2) s(-1) Pa-1 and good selectivities for hydrogen over CH4, CO and CO2 of over 1500 at 873 K. (C) 2007 Elsevier B.V. All rights reserved.