Tetraethoxysilane (TEOS) was introduced into the pores of porous glass and decomposed on the pore walls at 473 K. At the same time, a gas separation experiment using a helium-oxygen mixture was carried out. The permeation rate of each component began to decrease just after TEOS introduction, but reached a stable value after 35 hours in spite of the presence of TEOS. Just after TEOS introduction the separation factor was 3, the value expected in terms of Knudsen's equation; however, it began to increase rapidly after 30 hours and then reached a stable value of 6. Although TEOS introduction was stopped after 67 hours, both permeation rates and the separation factor remained unchanged. After this modification, another gas separation experiment with a helium-oxygen mixture was carried out, with varying temperature and composition on the upstream side. The dependence of helium permeation rate on temperature was the reverse of that through the original porous glass. The separation factor was improved over a wide range of temperature and composition, and the dependence on these parameters also changed. These results suggest that a different permeation mechanism was operating in the pores controlled finely by the modification.