The fracture behaviour of two polymer films (100 mu m thick) has been investigated according to the essential work of fracture (EWF) approach in order to assess the validity of the linear relationship between the specific total work of fracture and the ligament length, postulated to exist by the EWF theory, and examine the transition from plane strain to plane stress conditions. Double edge notched specimens loaded in tension were used; ligaments were varied in the range 0.3 to 45 mm. No linear relationship was found and it was confirmed that a power law can be very accurately fitted to the experimental data. An annealed copper film (100 mu m thick) exhibited the same pattern of behaviour.