Gas　separation　membranes　were　fabricated　with　varying　trimethylmethoxysilane　(TMMOS)/tetraethoxy　orthosilicate　(TEOS)　ratios　by　a　chemical　vapor　deposition　(CVD)　method　at　650　degrees　C　and　atmospheric　pressure.　The　membrane　had　a　high　H-2　permeance　of　8.3　x　10(-7)　mol　m(-2)　s(-1)　Pa-1　with　H-2/CH4　selectivity　of　140　and　H-2/C2H6　selectivity　of　180　at　300　degrees　C.　Fourier　transform　infrared　(FTIR)　measurements　indicated　existence　of　methyl　groups　at　high　preparation　temperature　(650　degrees　C),　which　led　to　a　higher　hydrothermal　stability　of　the　TMMOS-derived　membranes　than　of　a　pure　TEOS-derived　membrane.　Temperature-dependence　measurements　of　the　permeance　of　various　gas　species　were　used　to　establish　a　permeation　mechanism.　It　was　found　that　smaller　species　(He,　H-2,　and　Ne)　followed　a　solid-state　diffusion　model　while　larger　species　(N-2,　CO2,　and　CH4)　followed　a　gas　translational　diffusion　model.