Na+ and Cl- transport across 2.4- and 5.2-μm-thick polyimide (PI) PMDA/ODA and 7.5-μm-thick du Pont Kapton films was studied in a diffusion cell using ion-selective electrodes. The temperature was 30°C. Diffusion coefficients Dm (in cm2/s), permeability coefficients P (in cm2/s), and distribution coefficients K were obtained by applying the time lag analysis on the ion uptake transient in the acceptor compartment of the diffusion cell. Both Na+ and Cl- were observed to diffuse across PI. Diffusion coefficients varied in the range 10-12-10-13 cm2/s whereas ionic permeability results showed more scatter ranging from 1 X 10-11 to 8 X 10-14 cm2/s. The latter discrepancy was ascribed to diffuse heterogeneities on the order of 0.5 μm. No significant ionic selectivity between Na+ and Cl- was observed. This result indicates the absence of ionic fixed charge (unreacted polyamic acid) in the polymeric matrix. An attempt is made to determine the effect of water sorption mode and contaminants in the film on the ionic permeabilities data.