Small- and wide-angle X-ray scattering patterns were measured in real time during insitu deformation of isotropic polyethylene at room temperature. A linear low-density polyethylene (LLDPE) was compared with a high-density polyethylene (HDPE) of approximately the same molecular weight. SAXS showed that the start of cavitation in the HDPE coincided with the onset of a stress-induced martensitic transformation detected by WAXS at a low macroscopic strain (similar to 0.06). It is proposed that the transformation mechanism was the T2(2) mode. It was found that in the HDPE possible lamellar stack rotation occurred whereas in the LLDPE interlamellar shear was active. Altering the thermal history to change the degree of crystallinity and lamellar population altered the strain at which the martensitic transformation occurred. In the annealed LLDPE a higher crystallinity delayed the onset of the transformation by allowing a greater amount of deformation to occur via other deformation mechanisms. In annealed HDPE, however, the transformation was activated at lower strains since less deformation of the amorphous component was possible.