The plane-stress fracture toughness of a biaxial-oriented, filled poly(ethylene terephthalate) (BOPET) film was determined by the essential work of fracture concept using tensile-loaded deeply double-edge notched (DDEN-T) specimens. Results suggested that the essential work of fracture (w(e)) is a material parameter, whereas the non-essential or plastic work (beta w(p), where beta is the shape factor of the plastic zone) depends on the testing conditions. Various approaches were used to assess w(p) either explicitly (uniaxial tensile and trouser tests) or via estimation of beta [by infra-red thermography (IT)]. IT seemed to be the most reliable technique for this purpose. IT also answered the question whether necking precedes crack growth or these processes occur simultaneously during specimen loading. The out-of-plane type deformation (mode III) in the trouser test resulted in markedly lower w(e) and higher w(p) values than those determined by in-plane (mode I) tests on DDEN-T specimens. The difference was attributed to the microstructure-related anisotropy of the PET film caused by biaxial drawing.