We prepared poly(p-phenylene pyromellitimide) (PMDA-PDA), poly(p-phenylene 4,4'-hexafluoroiso-propylidene diphthalimide), and their copolyimides with various compositions to explore the relationship between the water sorption and structure. The water sorption behaviors were gravimetrically investigated as a function of composition and temperature and interpreted with a Fickian diffusion model in films. Overall, the water sorption behaviors were strongly dependent on the changes in morphological structure, which originated from the variations in composition. When the content of the bulky hexafluoroisopropylidene group (6FDA) was increased, the water uptake decreased from 5.80 to 3.18 wt %, whereas the diffusion coefficient increased from 3.6 X 10(-10) to 11.3 X 10(-10) cm(2)/s. The relatively high water uptake in the PMDA-PDA polyimide film was successfully healed by the incorporation of 6FDA, which may have resulted from the increases in the intermolecular packing order and hydrophobicity. The degree of orientation and crystallinity, which are in-plane characteristics, were directly correlated to the diffusion coefficient and activation energy in the polyimide film. (C) 2003 Wiley Periodicals, Inc.