The effect of CO2-philic additive polyethylene glycol (PEG) 200 in Matrimid 5218 on the separation performance of prepared membranes was evaluated in a binary gas mixture. Matrimid/PEG 200 flat-sheet blended membranes with low PEG concentrations were prepared by the dense film-casting method. Pure Matrimid and blended membranes were characterized by FTIR spectroscopy, scanning electron microscopy, differential scanning calorimetry, thermogravimetric analysis, and permeation measurements. The addition of 4-5 % of PEG enhanced considerably the CO2 permeability of the Matrimid matrix. The best formulation, Matrimid/PEG 200 (96/4), showed in comparison to pure Matrimid a more than threefold increase in CO2 permeability and an increase in separation factor of about 40 %.