The decohesion of high purity Cu-sapphire interfaces has been studied by using double cleavage drilled compressive (DCDC) specimens, accompanied by in situ optical observations of the crack extension mechanisms. The Cu layer thickness was varied between 10 and 100 µm. Decohesion occurred along the interface subject to a resistance, ⌫ R (⌬a), that rises with crack extension. The magnitude of the resistance was determined to be much larger (by a factor 4) for specimens with the thicker (100 µm) Cu layer. The observations revealed that the rupture occurred by a ductile mechanism involving void formation at the interface, followed by plastic void growth in the Cu, and subsequent coalescence by the separation of the interface between voids. Aspects of the failure sequence change with the Cu layer thickness. These changes, as well as the difference in the toughness, are attributable to a transition in constraint between the thin and thick layers. Namely, the thin layer develops high constraint (relative to the thicker layer) that elevates the mean stress ahead of the crack and causes the failure process to progress to a location further from the crack front. The measured trends in ⌫ R (⌬a) for the two different layer thickness are shown consistent with models of the constraint transition for interface failure by ductile mechanisms.