The permeations of simple permanent gases and their mixtures through plasma-polymerized composite membranes were studied. Composite membranes were prepared by the plasma polymerization of fluorocarbon monomers such as pentafluorotoluene (PFT) and pentafluoropyridine (PFP) onto porous Celgard. For pure gases, the permeability coefficients were mainly affected by diffusivity rather than solubility. The permeability coefficient decreased as the kinetic molecular diameter of the penetrant molecules increased, and the permeability coefficients were independent of pressure. For mixed gases, the permeability coefficient was not affected by the composition of penetrants for the whole range of composition.