The impact behaviour of three linear low density polyethylenes (LLDPE) has been studied using the energy partitioning methodology recently developed in this laboratory. The aim was to elucidate the origin of the high toughness of this class of polymers and to establish whether a rubber toughening mechanism is involved. It has been found that LLDPEs have an unexpectedly high intrinsic toughness, and that there is a significant ductile contribution to the total fracture energy from shear lip energy absorption. No evidence of a conventional rubber toughening mechanism has been found. However, the shear lip area (and hence this specific component of the total fracture energy) increases with the concentration of the segregated ’rubbery’ phase.