The effects of thermal treatment from 180 degrees C to 1150 degrees C on the gas transport properties of Porous silica membranes were systematically studied for various gases. The permeance of all gases, except for CO2, has it maximum at 800 degrees C. The CO2 permeance was constant from 180 degrees C to 600 degrees C and then decreased monotonically. Membranes thermally treated at 1150 degrees C did not exhibit any gas permeation because of pore collapse. The gas transport behavior follows a combination of Knudsen diffusion and surface diffusion for all gases tested except for carbon dioxide. The permeation of carbon dioxide is strongly affected by capillary condensation. We propose a new transport model composed of two components; that is, the Knudsen diffusion factor, alpha, and the Surface diffusion factor, beta. A transition was observed for alpha and beta at around 800-900 degrees C, which is close to the strain point of the membrane. This transition treatment temperature can be correlated with the changes in gas permeance. The model allows qualitative evaluation of gas transport through porous membranes regardless of their actual microporous structures.