The compression behaviour of a range of unoriented linear and branched polyethylenes was investigated using the technique of simultaneous small-and wide angle X-ray scattering during deformation. Interlamellar deformation mechanisms, including simultaneous shear and rotation of lamellar stacks, were followed by crystallographic deformation mechanisms, including (100)[001] chain slip and a martensitic transformation. Immediate post-yield deformation behaviour involved a reconstruction of the lamellar microstructure, the deformation of which was responsible for strain hardening. The reconstruction process, which involved extensive lamellar rearrangement, was aided by the increased amount of amorphous material in the less crystalline linear low density branched polyethylenes. Branch length, however, was found to be unimportant.