The mechanisms of mode I crack propagation along Au/sapphire interfaces in both humid and dry environments have been investigated. Interface failure occurs by bond separation at the interfaces accompanied by substantial deformation of the Au along intersecting slip planes. In a dry environment, crack extension occurs by interfacial decohesion at multiple sites ahead of the main crack front. The associated fracture energy Γ i is of order 50 J m - 2 . These debonds initiate from interfacial defects. The main crack experiences extensive plastic blunting; whereas debonds ahead of the crack have diminished blunting and propagate, subject to a rising resistance curve. Because of the small blunting displacements, the stresses ahead of the debond are sufficient to cause rupture of the interface bonds. In a humid environment, the crack propagation mechanism changes. In this case, the crack front itself extends, when Γ i 10 J m - 2 , subject to small blunting displacements.