Hydrogen-permselective SiO2 films were formed within the pores of glass support tubes by chemical vapor deposition from tetraethylorthosilicate (TEOS). Deposition Of SiO2 involved the decomposition of TEOS both in the absence of and in the presence of oxygen at 200-700-degrees-C and 1 atm. The properties of the silica films produced were greatly affected by the deposition conditions, the temperature, the contact scheme of reactants, and the amount of SiO2 deposited. In the absence of oxygen, the films produced appeared to be relatively labile and their stabilities were dependent on the deposition conditions. In the presence of oxygen, however, stable and highly H-2-selective membranes were synthesized at temperatures above 300-degrees-C by one-sided deposition; and unstable films were obtained at high concentration of TEOS by opposing-reactant deposition. The permeation rates of H-2 at 600-degrees-C through the SiO2 membranes were 0.20-0.42 cm3 (STP)/cm2 -min-atm and the H-2:N2 permeation ratios varied between 500 and 3000. The SiO2 films Produced using TEOS and oxygen were less dense than those deposited by SiCl4 hydrolysis. Thus the former membranes had higher H-2 permeation rates but had lower H-2 selectivity as the pores of the substrate were plugged with SiO2.