We have measured the fracture toughness, G(o), of an interface between polystyrene (PS) and poly (2-vinylpyridine) (PVP) reinforced with triblock copolymers (PVP-b-dPS-b-PVP) as a function of the areal chain density, Sigma, of the copolymers at the interface. The failure mechanisms of the interface are studied by transmission electron microscopy and forward recoil spectrometry. For triblock copolymers with long PVP blocks (N-PVP > N-ePVP, where N-ePVP is the entanglement polymerization index of PVP), a transition from chain scission at low Sigma to crazing at high Sigma* is observed. By comparing the areal chain density Sigma* for the transition from chain scission to crazing for the triblock copolymers (Sigma* = 0.015 chains/nm(2)) to that for the diblock copolymers, PVP-dPS (Sigma* = 0.03 chains/nm(2)), we show that most of the triblock copolymers form a "staple" structure at the interface with the dPS block making a loop on the PS side of the interface and the PVP ends anchoring the "staple" in the PVP side. As a result of the "staple" structure, the saturation areal chain density of the triblock copolymer (Sigma(sat)) at the interface is half of that for the diblock copolymer of similar molecular weight. For Sigma < Sigma(sat) in the crazing regime, the fracture toughness of the interface is controlled by the areal joint density, Sigma(cross), where Sigma(cross) is the number density of the copolymer excursions across the interface. For Sigma > Sigma(sat), the triblock copolymers appear to reinforce the craze fibrils at the crack tip better than the corresponding diblock copolymers, leading to an interface fracture toughness approaching that of the PS homopolymer itself. For a triblock copolymer with short PVP blocks (N-PVP < N-ePVP), there is a transition in the fracture mechanism from pull out of the PVP block to crazing with increasing Sigma. Short triblock copolymers can form two chain conformations : one in which two PVP blocks anchor the copolymer on the homopolymer PVP side (staple structure) and one in which one PVP block anchors the copolymer on the PVP side (tail structure) of the interface. Comparison of G(c) between the triblock copolymer and the corresponding diblock copolymer is made. The larger G(c) values of the triblock copolymer reinforced interface in the crazing regime are observed as a result of enhancement in entanglements between the dPS loops of the triblock copolymer and the homopolymer PS.