An aromatic polyamide with long alkyl side chain, A-C12, was synthesized from the condensation of 1,4-didodecylester of pyromellitic acid and p-phenylenediamine to clarify the relationship between its higher order structure and gas sorption properties. A-C12 formed a layered structure composed of alternating rigid aromatic main chain layers and flexible dodecyl side chain layers. This polyamide could be imidized by the usual imidization method. Imidized A-C12 could not keep the layered structure of A-C12. CO2 sorption of A-C12 occurred only in the side chain layer region, which is almost a liquid-like environment, and the sorption isotherms obeyed Henry's law. This result also indicates that the main chain layer is very dense and cannot sorb CO2. On the other hand, imidized A-C12 showed a large sorption amount of CO2 and dual mode sorption, despite high density, meaning imidized A-C12 does not contain a layered order structure but an amorphous glassy state. (C) 2005 Wiley Periodicals, Inc.