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A new technique to modify the zeolite molecular sieve membrane has been developed. Zeolite membranes have polycrystalline structure, and penetrant molecules can transport through zeolite crystals and among crystal faces. When the intercrystalline region are plugged, the zeolite crystal pores are the only pathway for membrane transport. That is a promising way to obtain a good molecular sieving effect. Counterdiffusion CVD technique was carried out to fill the intercrystalline region. Tetraethyl orthosilicate (TEOS) was employed as a silica source, and O 3 was used as an oxidizing agent; these reactants form amorphous silica by CVD reaction. TEOS hardly penetrated into zeolitic pores because of its molecular size, and zeolitic pores may not have been changed by the reaction. Thus, only the intercrystalline region was filled due to the silica formation. The CVD-modified silicalite membrane produced a n-butane selectivity of 87.8 over isobutane at 288 K.