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Silica-organic hybrid membranes were successfully prepared on -alumina substrates by using a counter diffusion CVD method. Propyltrimethoxysilane (PrTMOS) was employed as a silica source, and O 3 was used as a reactant. Deposition temperatures of the CVD were very important parameters for the membrane properties. C 3 H 6 /C 3 H 8 peremeance ratio was 12.1 at 320 through a membrane prepared at 320 . C 3 H 6 /C 3 H 8 peremeance ratios were proportional to N 2 /SF 6 permeance ratios showing that the C 3 H 6 /C 3 H 8 selectivity are due to a molecular sieving mechanism. The decomposition mechanism of propyl groups on silica surface was discussed by using powders prepared by hydrolysis of PrTMOS. Propyl groups were partially decomposed under 320 of an O 3 atmosphere. This indicates that the obtained membrane was a silica-organic hybrid membrane.

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