A stable silica membrane having excellent H-2/N-2 permeance ratio (over 1000) was prepared by the counter diffusion chemical vapor deposition method using tetramethyl orthosilicate (TMOS) and O-2 as reactants at 873 K. TMOS and O-2 were provided in the opposing geometry of the Substrates. and silica layer was deposited in the substrate pores. Apparent activation energies through the silica membranes increased with increasing deposition temperatures. The activation energy of H-2 was ca. 20 kJ mol(-1) through the membrane. H-2 permeance at 873 K permeation test was 1.5 X 10(-7) mol m(-2) s(-1) Pa-1. H-2/N-2 permeance ratio was kept for 21 h under the typical steam-reforming conditions of methane for a membrane reactor (76 kPa of steam at 773 K). (c) 2004 Elsevier B.V. All rights reserved.