The surface fluorination of both thick isotropic films and composite membranes formed from poly (vinyltrimethylsilane) (PVTMS) is described. For films ranging in thickness from 23 to 200 mum, fluorination treatment reduced the permeability coefficients for the gases N2, O2, CH4, CO2, Kr and Xe while the permeability of He was not affected. The ideal selectivities for He, CO2, Kr and Xe relative to N2 or CH4 were increased, but the ratio for the O2/N2 Pair remained unchanged. Repeated fluorination at increasingly harsh fluorination conditions progressively lowered the permeability and increased the selectivities still further. Effective diffusivity coefficients for the entire treated film were found to be more responsive to fluorination than the solubility coefficients. For composite membranes, comprising a thin (approximately 0.5 mum) selective layer of PVTMS, two sets of fluorination conditions were considered : variable treatment time at a constant fluorine feed concentration of 0.02% and different feed concentrations at a constant treatment time of 2 min. In the absence of fluorination damage, the selectivity generally increased as a function of treatment time while the permeance (P/l) was reduced. Fluorination damage to some membranes caused a decrease in both selectivity and permeance, but these defects could be caulked by applying a surface layer of poly(dimethylsiloxane).