The deformation behaviour of four different grades of polyethylene has been examined as a function of the morphology produced by pressure annealing in the vicinity of the orthorhombic-hexagonal phase boundary. It is concluded that annealing within the hexagonal phase, to produce a "chain-extended" morphology, is appropriate for high molecular weight material as a precursor step in the production of high modulus by solid phase deformation. In low molecular weight material of the same morphology solid phase deformation does not produce improvements in mechanical behaviour. These results are interpreted in terms of the competition between increasing crystal size and the decrease in the coherence of the molecular network brought about by the pressure annealing. The critical parameter is the ratio of lamellar size to number average molecular chain length, rather than lamellar morphology. When this parameter is greater than 0.5 the molecules are incapable of completing two crystalline traverses and the network is affected detrimentally to the extent that it no longer holds the material together.