Samples of a ``high‐density'' polyethylene (&rgr;=0.964 g/cc) irradiated in a nuclear reactor to thermal neutron doses as high as 2.9×1018nvt and of ``low‐density'' polyethylene (&rgr;=0.915 g/cc) subjected to gammaray bombardment from a Co60source up to a dosage of 1×109rep have been investigated by dynamic mechanical methods from 80°K to 450°K at audio‐frequencies. Degrees of cross‐linking were estimated from the observed changes in resonance frequency at temperatures where the specimens were ``rubber‐like.'' Densities and x‐ray crystallinities at 25°C were also measured.For the pile‐irradiated ``high‐density'' polyethylene the &ggr; peak found around 165°K shows a marked increase in height and area at dosages above 1018nvt. The &agr; peak at about 390°K decreases markedly upon receiving a thermal neutron dose of 0.6×1018nvt. Further changes in this absorption with higher dosages are obscured by the appearance of a &bgr; peak around 330°K which is almost absent in the unirradiated material; the appearance of the &bgr; region is attributed to the creation of branch points and cross‐link points in the essentially linear polymer by the irradiation.The irradiated ``low‐density'' specimens exhibit three damping peaks. Over the dosage range studied the &ggr; peak (165°K) increases in height and area, the &bgr; peak (270°K) shifts to higher temperatures, and the &agr; peak (360°K) shifts to lower temperatures with increasing dose. The cross‐linking degree is not proportional to dose and to the highest dosages studied, there is very little, if any, reduction in x‐ray crystallinity.