The energy release rate of the ENF adhesive joint specimen is derived. The resulting formula accounts for the presence of an adhesive layer with a possibly non-linear constitutive relation. The actual form of the constitutive relation need not to be known a priori. It is shown that measurements of both the applied load and the adhesive deformation at the crack tip are needed to determine the instantaneous value of the energy release rate. Experimental results show that the influence of the deformation of the adhesive layer can be substantial. The stability of the ENF-specimen is studied and a method to estimate the critical value of the crack length, accounting for the flexibility of the adhesive layer, is presented. In the analysis, the adherends are modelled as elastically deforming Euler–Bernoulli beams and the intermediate adhesive layer is modelled as a material surface transmitting shear stress between the adherends.