Poly(4,4’-oxydiphenylene pyromellitimide) (PMDA-ODA) films of 10-109 mu m thickness were prepared from its poly(amic acid) precursor by thermal imide-ring-closure formation at various temperatures. Water sorption in the films was measured at 25 degrees C over 22-100% relative humidity using an electromicrobalance. Water diffusion in all the films was a nearly Fickian process despite the morphological heterogeneity due to the ordered and less ordered phases. Depending upon humidity, film thickness and imidization history, the diffusion coefficient and water uptake varied in the ranges of 1 x 10(-9) to 3 x 10(-9) cm(2) s(-1) and 0.4 to 4.5 wt%, respectively. Overall, both the diffusion coefficient and the water uptake increased with increasing humidity and film thickness, but decreased as the imidization temperature and time increased. The water sorption results were interpreted by consideration of morphological variations (molecular order, chain orientation and microvoids) due to film thickness and imidization history.