Cellulose triacetate (CTA) is a workhorse polymeric membrane material for industrial CO 2 /CH 4 separation. This study investigates the doping of CTA with ionic liquids (ILs) to reduce the crystallinity of the CTA and enhance its affinity with CO 2 , thus increasing CO 2 permeability and CO 2 /light gas selectivity. CTA films doped with 1-ethyl-3-methylimidazolium tetrafluoroborate ([emim][BF 4 ]) and 1-ethyl-3-methylimidazolium dicyanamide ([emim][dca]) were prepared; and the effect of IL loading on properties of the CTA, such as crystallinity, density, degradation temperature, glass transition temperature, and gas transport properties, were determined. The IL doping reduces the glass transition temperature and degree of crystallinity of CTA, increasing gas diffusivity and permeability. The IL doping also increases CO 2 /CH 4 solubility selectivity and CO 2 /N 2 permeability selectivity, due to the affinity of these ILs with CO 2 , compared to light gases such as CH 4 and N 2. This study demonstrates a promising route in manipulating the morphology and gas transport properties of semi-crystalline polymers by doping with ILs.