Fracture behavior of binary blends comprising styrene-butadiene block copolymers having star and triblock architectures was studied by instrumented Charpy impact test. The toughness of the ductile blends was characterized by the dynamic crack resistance concept ( R curves). While the lamellar thermoplastic star block copolymer shows elastic behavior ( small scale yielding and unstable crack growth), adding 20 wt% of a triblock copolymer ( thermoplastic elastomer, TPE) leads to a strong increase in crack toughness. The stable crack propagation behavior of these blends was described by the crack resistance curve ( R) concept of elastic-plastic fracture mechanics. This concept allows the determination of fracture mechanics parameters as resistance against stable crack initiation and propagation. Two brittle to tough transitions (BTT) are observed in the binary block copolymer blend: BTT1 at 20% TPE and BTT2 at about 60% TPE. The strong increase of toughness at 60 wt% TPE indicates a 'tough/high-impact' transition as a measure for the protection against stable crack initiation.The kinetics of stable crack propagation is discussed with respect to deformation mechanisms and crack-tip blunting behavior. The analysis of fracture surface by SEM revealed three different types of deformation mechanisms depending on the weight fraction of TPE: coalescence of microvoids ( similar to semicrystalline polymers), shear flow ( typical of many amorphous polymers like polycarbonate) and tearing ( similar to elastomers). Our investigations on nanostructured binary block copolymer blends show new possibilities to tailor the toughness of polymer materials associated with complex morphology-toughness correlations. This may lead to new materials concepts for toughened nanostructured polymers, which still maintain excellent transparency. (C) 2004 Kluwer Academic Publishers.