The　effect　on　the　gas　permeance　properties　and　structural　morphology　of　the　presence　of　methyl　functional　groups　in　a　silica　membrane　was　studied.　Membranes　were　synthesized　via　chemical　vapor　deposition　(CVD)　at　650　degrees　C　and　atmospheric　pressure　using　three　silicon　compounds　with　differing　numbers　of　methyl-　and　methoxy-functional　groups:　tetramethyl　orthosilicate　(TMOS),　methyltrimethoxysilane　(MTMOS),　and　dimethyldimethoxysilane　(DMDMOS).　The　residence　time　of　the　silica　precursors　in　the　CVD　process　was　adjusted　for　each　precursor　and　optimized　in　terms　of　gas　permeance　and　ideal　gas　selectivity　criteria.　Final　H-2　permeances　at　600　degrees　C　for　the　TMOS-,　MTMOS-,　and　DMDMOS-derived　membranes　were　respectively　1.7　x　10(-7),　2.4　x　10(-7),　and　4.4　x　10(-8)　mol　center　dot　m(-2)center　dot　s(-1)center　dot　Pa-1　and　H-2/N-2　selectivities　were　990,　740,　and　410.　The　presence　of　methyl　groups　in　the　membranes　fabricated　with　the　MTMOS　and　DMDMOS　precursors　was　confirmed　via　Fourier-transform　infrared　(FTIR)　spectroscopy.　From　FTIR　analysis,　an　increasing　methyl　signal　in　the　silica　structure　was　correlated　with　both　an　improvement　in　the　hydrothermal　stability　and　an　increase　in　the　apparent　activation　energy　for　hydrogen　permeation.　In　addition,　the　permeation　mechanism　for　several　gas　species　(He,　H-2,　Ne,　CO2,　N-2,　and　CH4)　was　determined　by　fitting　the　gas　permeance　temperature　dependence　to　one　of　three　models:　solid　state,　gas-translational,　or　surface　diffusion.