Hydrogen-permselective membranes were prepared by atmospheric pressure chemical vapor deposition (APCVD) of SiO2 layers in porous Vycor tubes. The deposition was carried out by passing the reactants SiCl4 and H2O through the bore of the support tubes at temperatures ranging from 600 to 800-degrees-C. The deposit layers were examined by TEM, SEM, and EPMA. When the deposit was confined inside the pores of the Vycor substrate, the membranes were mechanically stable but when it extended substantially outside of the porous matrix the stresses induced by thermal cycling led to crack formation and propagation. Electron microscopy revealed that the SiO2 deposit density is maximum in a region approximately 0.5 mum thick adjacent to the bore surface and gradually declines to zero within a depth of approximately 10 mum from the surface. The thin region of maximum deposit density is responsible for permselectivity, for it essentially blocks the permeation of nitrogen and larger molecules while allowing substantial permeation of hydrogen. This region contains approximately 10% by volume trapped voids and as a result has relatively high permeability as suggested by the percolation theory. Annealing at high temperatures causes densification of the deposited material as evidenced by increased activation energy for H-2 permeation and correspondingly reduced permeance. The presence of H2O vapor accelerates the densification process. The densified membranes had a H-2 permeance as high as 0.1 cm3 (STP)/min-atm-cm2 at 500-degrees-C and a H-2/N2 permeance ratio above 500.