Sorption and transport behavior of water vapor in two copolyimide dense films and their asymmetric capillary membranes were studied at 30degreesC. Water vapor permeation and separation behavior in the copolyimide dense and asymmetric membranes were also investigated in vapor feeds of pure water, 1-propanol/water, and acetic acid/water with concentration of 10 wt.% of organic compounds at 85-135degreesC. Experimental results showed that a solvent-resistant, defect-free asymmetric polyimide membrane could be prepared from its precursor poly(amic acid) (PAA) by a dry/wet phase inversion process to obtain a PAA asymmetric membrane first that was then thermally imidized. The skin structure of the asymmetric polyimide membrane differed somewhat from that of polyimide dense films. Water sorption behavior in the polyimide dense and asymmetric membranes could be successfully interpreted by the Guggenheim-Anderson-de Boer model. The sorption and transport behavior of water vapor in the skin layer of an asymmetric membrane could be approximately expressed by the effective thickness of that of a homogeneous dense membrane, which determines the membrane permeation and separation Properties. (C) 2004 Elsevier B.V. All rights reserved.