Carbon dioxide and methane permeabilities have been measured for dense film and asymmetric membrane prepared from an aromatic polyimide (6FDA/DMB). Selectivities for CO2/CH4 of up to 55 were determined for mixtures of these gases. Permeability of gases through the membrane was found to be dependent upon the carbon dioxide and higher hydrocarbon concentrations that can also be present in natural gas streams. In addition to carbon dioxide having an impact on methane permeability, the presence of hexane or toluene cut CO2/CH4 selectivities in half. Lowered selectivities from ideal test conditions are a result of plasticization of the polyimide by these components. Results for the polyimide are contrasted with values obtained from cellulose acetate films which are less impacted by hydrocarbon impurities. The polyimide depends more upon diffusivity factors than cellulose acetate to generate high selectivity. Thermal and physical properties of 6FDA/DMB polymer and membrane are also described,