The fracture toughness of an amorphous copolyester (COP) of different sheet thickness (0.5, 3 and 6 mm) was determined by the essential work of fracture (EWF) concept using tensile-loaded deeply double-edge notched (DDEN-T) specimens. It was shown that this COP meets the basic requirement of the EWF concept, viz, full ligament yielding (marked by a load drop in the load-displacement curve) prior to the crack growth, in the thickness range studied. Based on the well-resolved yielding both the specific essential (w(e)) and non-essential work of fracture (w(p)) were split in the contributing terms related to yielding (w(I)), and necking and fracture (w(II)). It was found that w(e) is likely to be independent on the thickness range when plane stress conditions prevail, and thus represents a material parameter. This finding is at odds with previous results suggesting that w(e) is thickness dependent. The development and size of the plastic zone were studied by light microscopy and infra-red thermography. The latter technique overestimated the plastic zone in thicker sheets by not differentiating between pure and diffuse yielding.