Iodine diffusion in ion-modified polyethylene (PE) using the Rutherford Backscattering method (RES) has been studied. PE was irradiated by N+, Ar+ and As+ ions with an energy of 150 keV and doses of 1 X 10(13)-1 X 10(15) cm(-2). Iodine diffused in ion-modified PE from vapor at 90 degrees C. Iodine’s concentration profile changed its shape dramatically for ion doses over 1 X 10(14) cm(-2) when it showed two maxima. A similar profile was exhibited by oxygen, which diffused in PE on implantation. Iodine’s concentration dropped in the layer where the most significant polymer carbonization occurred. This range was found ahead of the implanted ions concentration. Iodine diffusion was most intensive for lower ion doses (less than or equal to 1 X 10(14) cm(-2)) while for higher doses it was substantially slower due to PE carbonization. The reason was the lower free volume in the PE carbonized layer as compared with the layer where the polymer’s degradation was not reflected in such a significant increase in carbon content.