Polyimide layers approximately 10 μm thick on Si wafers were ion beam etched (IBE) by Ar. Ion energies up to 1000 eV and beam current densities as high as 0.3 mA/cm2were used to a total dose of 54 mC/cm2. After etching, the samples were exposed to iodine vapor for fixed periods of time. The diffusion of iodine into the samples was used to probe for ion induced changes in the polyimide structure. The concentration of the diffused iodine was measured as a function of depth by Rutherford backscattering spectrometry. For the IBE samples the surface concentration of iodine was markedly decreased. The iodine diffusivity in the near surface region of thickness 0.2 μm was reduced by two orders of magnitude. These results indicate that etching appears to cause modification of the polyimide film at depths far greater than the range of the incident ions or their secondary electrons in polyimide.