AbstractThe temperature, strain‐rate, and pressure dependences of the yield stress have been determined for two polyethylene homopolymers of differing molecular weights and for a polyethylene copolymer. Samples were prepared by slowly cooling from the melt, and also by quenching in order to assess the effects of morphology on the yield behavior. The data have been analyzed on the basis of two Eyring processes acting in parallel. Comparison of all data sets reveals the existence in general of three distinct activated processes each with its own temperature, strain‐rate, and pressure dependences. The relative contribution of each process is dependent on the molecular weight, morphology, etc. Also discussed is the relation of these three yield processes to the well‐known loss processes of linear viscoelast