Toughness and deformation capacity of six polyethylene copolymers in plane-strain fracture were characterized using the concept of essential work of fracture (EWF). Two types of regression analysis were considered: one based on the traditional, total work of fracture, while the other on the energy partition to extract the portion that is relevant to the plane-strain fracture. In particular, the latter analysis excludes energy that is for the final stretch of the surface flanks and produces toughness values that are smaller than those determined based on the total work of fracture. The study found that two types of analysis rank the copolymers in different orders. Based on the results from the energy-partition approach, the article discusses the influence of material characteristics (molecular weight, branch concentration, density, etc.) on the plane-strain fracture toughness of the polyethylene copolymers. POLYM. ENG. SCI., 50:530-535, 2010. (C) 2009 Society of Plastics Engineers