The fracture toughness of Mo in Cu/Mo bilayers on polyimide was assessed with in situ X-ray diffraction during uniaxial tensile straining. The fracture resistance of Mo acting as an adhesion layer greatly depends on the thickness of the Cu layer exhibiting a toughening effect with increasing Cu layer thickness. In contrast, the presence of the Mo interlayer greatly decreases the apparent K _Ic of the Cu layers. The quantification of K _Ic for Mo with a Cu top layer provides further evidence that when brittle layers are used, a thicker ductile layer is advantageous to create fracture resistant stretchable systems.