Model experiments, involving glass slides bonded by a thin adhesive layer, have been used to investigate some aspects of interfacial failure under conditions of mixed-mode loading. These experiments illuminate the significance of the choice of scale at which an interface is discussed. When the specimens were viewed as being elastically homogeneous entities with a plane of weakness, a correlation was observed between the apparent fracture resistance of the interface and the degree of mixed-mode loading. However, at a finer scale of examination, it was appreciated that this correlation could be attributed in part to a subtle change in the actual failure mechanism. © 1990.