The failure mode of a number of polyethylenes has been studied under predominantly plane strain conditions. Square section samples, notched on all sides, have been tested in tension to failure over a range of crosshead speeds and temperatures. Integration under the subsequent load displacement curve has allowed the total energy, energy to peak load, and energy after peak load to be determined. The data have been analyzed in terms of the ratio of the energy after the peak load to the total energy. The results show that the material can change from brittle to ductile failure as a function of test speed. At a suitable temperature we have observed brittle-like failure at the highest and lowest test speeds and ductile failure at intermediate speeds. The resulting failure surface features correlate very strongly with the energy ratio analysis-flat smooth surfaces where low energy ratios are seen and large ductile tearing where high ratios are seen. The effect of molecular weight and polydispersity will be shown and possible failure mechanisms discussed.