A study was made on the permeation rate of pure gases including helium, hydrogen, oxygen, nitrogen, carbon dioxide and methane, and the separation of CO2/CH4 gas mixtures by a silicone-coated aromatic polyamide membrane. The ideal and actual separation factors of the polymer towards these gases were calculated. The membranes were prepared by the phase-inversion technique and coated with silicone rubber after drying. While the permeation rates of He, H-2 and CO2 were not affected significantly by coating, the permeation rates of N-2 and CH4 gases decreased to below the detectable limit of the flow meter. The permeation rate of O-2 decreased after coating. The actual separation factor for mixtures of CO2/CH4 varied from 64 to 166, one of the highest separation factors reported in the literature.