Thin SiO2 layers were deposited on the surface of porous Vycor glass by alternating vapor phase reaction with SiCl4 and H2O. The membranes prepared by this technique had a H-2 permeance of 0.3-0.4 cm(3)(STP)/(min.cm(2.)atm) and a H-2:N-2 selectivity of 500-1000 at 600 degrees C. The SiCl4 dosage at each silylation cycle, the concentration of initial surface OH groups, and the reaction temperature significantly influence the deposit layer thickness. After two weeks of heating at 550 degrees C under 3 atm of water vapor, the membrane H-2 permeance decreased by about 20%, and the selectivity increased to more than 2000. The membrane properties after this hydrothermal treatment are superior to those of membranes prepared earlier by one-sided (steady flow) deposition. A simple model incorporating diffusion and surface reaction was used to study the effect of various parameters on the formation of the deposit layer.