Asymmetric structures were fabricated by depositing Y2O3-doped SiO2 (Si/Y) membranes onto -Al2O3 supported by tubular -Al2O3. The thickness of the Y2O3-doped SiO2 deposits was approximately 100 nm. The deposits/membranes have micropores with a pore diameter similar to<0.40-0.55 nm. Pore size distribution measurements were conducted directly on the membranes before and after hydrothermal treatment with a nano-permporometer. The gas permeance properties of the membranes were measured in the temperature range 100 degrees C-500 degrees C. The Y-doped SiO2 membrane (Si/Y = 3/1) was found to exhibit asymptotically stable permeances of 2.39 x 10(-7) mol/m(2)/s/Pa for He and 6.19 x 10(-10) mol/m(2)/s/Pa for CO2, with a high selectivity of 386 (He/CO2) at 500 degrees C for 20 h in the presence of steam. The Y-doped silica membranes exhibit very high gas permeances for molecules with smaller kinetic diameters. The apparent activation energies of the H-2 permeance at 400 degrees C were 24.2 +/- 0.2 and 21.3 +/- 0.7 kJ/mol for SiO2 and Si/Y, respectively.