The changes in mechanical properties of polyethylene (medium density, MDPE) and poly(tetrafluoroethylene) (PTFE) induced by heavier ions such as He2+ 20 MeV, C5+ 220 MeV and O5+ 100 MeV ions were studied and compared with those induced by 2 MeV electrons. The changes in elongation at break were similar for exposures to the heavier ions and for 2 MeV electron irradiations. The mechanical properties of the two polymers therefore seem to have little LET dependence.