Composite membranes containing a thin-film layer of aromatic polyimides (PI) ensure an advantageous combination of selectivity and permeability in gas separation. A series of rigid-chain PI with different chemical structures were studied as a thin active layer. Composite membranes were prepared by coating a solution of poly(amic acid) (PAA) and an imidization catalyst on a poly(phenylene oxide) (PPO) support with pores filled by decane. The subsequent stage of solid-state catalytic transformation of the PAA/PPO membrane into the PI/PPO membrane determines the specific structure of the PI layer and the transport properties of the PI/PPO composite membranes. The structure of composite membranes was determined by scanning electron microscopy and analyzed in the terms of the resistance model of gas transport in composite membranes. (C) 2000 John Wiley & Sons, Inc.