Composite membranes combining polyaniline as an active layer with a polypropylene support have been prepared using an in situ deposition technique. The protonated polyaniline layer with a thickness in the range of 90200 nm was prepared using precipitation, dispersion, or emulsion polymerization of aniline with simultaneous deposition on top of the porous polypropylene support, which was immersed in the reaction mixture. Variables such as temperature, concentration of reagents, presence of steric stabilizers, surfactants, and heteropolyacid were found to control both the formation and the quality of the polyaniline layers. Both morphology and thickness of the layers were characterized using scanning electron microscopy. Selective separation of carbon dioxide from its mixture with methane is used to illustrate potential application of these composite membranes. (c) 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012