The fracture toughness of the interface between the high Tg thermoplastic poly (ether imide) (PEI) and the high crosslinking density thermosetting epoxy RTM6 is characterized as a function of contrasting curing cycles. The cracking resistance and crack paths are related to the morphology of the interphase region as well as with the local elastoplastic and fracture properties of the corresponding homogenous blends at various concentrations. The results from uniaxial compression and nanoindentation tests exhibit an unexpected softening of the blends at low PEI content. The results from three-point bending tests performed on the RTM6-PEI interphases show either a tough or brittle behavior depending on the width of the morphological gradient in the interdiffusion zone. In tough interphases, the propagation of the crack is trapped inside a wide morphological gradient as evidenced by the analysis of the fracture surfaces. This interfacial confinement of the cracks is understood from a synergistic coupling between a lower local strength, higher local fracture toughness and the morphology of the interface region.