The diffusion of NMP (n-methyl-pyrrolidinone) solvent in a semirigid rod-like PMDA-ODA (pyromellitic dianhydride-4,4’-oxydianiline) film coated on silicon is found to be case I diffusion at temperatures ranging from 30 to 90 degrees C by the use of a bending beam technique. The diffusion constant increases for the 7.4 mu m PMDA-ODA film (which was cured at 300 degrees C) from 3.3 to 318 X 10(-11) cm(2)/s as the diffusion temperature increases from 30 to 90 degrees C. The corresponding hygroscopic stress in the direction parallel to the film decreases with the increase of temperature, possibly due to the softening of the film at elevated temperatures. The diffusion mechanism, however, changes from case II to case I in a rigid rod-like PMDA-PDA (pyromellitic dianhydride-p-phenylenediamine) film when the diffusion temperature increases. The change in the diffusion mechanism occurs at a higher temperature for thinner films, presumably due to higher ordering and/or orientation in the films. The activation energy for NMP diffusion in the PMDA-ODA films markedly decreases from 93 to 59 kJ/mole as the film thickness increases from 2.2 mu m to 11.3 mu m. This may also be attributed to decreased ordering in thicker films.