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CRACK propagation in polymers is a topic of considerable importance to the technological application of these materials. In high-molecular-weight glassy polymers, crack propagation is normally assumed to require the breaking of covalent bonds in polymer chains that span the crack path, as opposed to chain slip and disentanglement. The fraction of such broken chains in a bulk sample would be very small, however, so that there is little direct evidence for this chain breakage1. Interfaces between immiscible polymers are often very weak, but can be strengthened by the presence of a diblock copolymer (a polymer in which each molecule consists of a block of one polymer bonded to a block of a different polymer) for which each of the two blocks is miscible with one of the homopolymers2. The diblock copolymer is thought to organize at the interface and so 'stitch' the homopolymers together. Here we show that crack propagation along the interface between the homopolymers involves breaking all the copolymer chains at a point in the region of the junction between the two blocks. Thus we demonstrate both that the block copolymer does indeed organize at the interface and that fracture involves the breaking of polymer chains. This information may suggest means by which the mechanical properties of polymer blends could be systematically improved.