The gas separation properties of the polycarbonate-polypyrrole mixed matrix membranes were evaluated based on the introduction of conducting polymer as powder fillers. The electrically conductive fillers are obtained from two main synthesis routes namely electrochemical and chemical methods. The permeation properties of polycarbonate-polypyrrole systems are highly dependent on the synthesis method (electrochemical or chemical) and also the membrane casting conditions (casting solvent type). The introduction of electrochemically synthesized polypyrrole fillers distorted the polycarbonate matrix by forming cavities which resulted in higher permeation rates of electrochemically synthesized polypyrrole-polycarbonate (ECPPY-PC) membranes compared to both pure polycarbonate (PC) and the chemically synthesized polypyrrole-polycarbonate (CPPY-PC) films accompanied with a loss in selectivities. On the other hand, the densified structure of CPPY-PC membranes resulted in an improvement of the selectivities of industrially important gas pairs (H-2/N-2, O-2/N-2 and H-2/CH4). 0 2003 Elsevier B.V. All rights reserved.