Polystyrene-poly(styrene-co-butadiene)-polystyrene triblock copolymers (PS-P(S-co-B)-PS) having different styrene contents (from 30 wt.% to 80 wt.%) in the statistical copolymer middle block and different block architectures (20-60-20 and 30-40-30) were characterised to study the influence of S/B middle block composition and segregation strength on the morphology and mechanical behaviour. The morphological investigations, i.e. TEM and SAXS, exhibited ordered lamellar and lamellar-like morphologies for both block architectures at low styrene contents between 30 wt.% and 50 wt.% in the S/B middle block. The increase in the styrene content in the middle block to 70 wt.% resulted in phase separated structures without long range order due to the enhanced miscibility between the PS and P(S-co-B) phase as observed from dynamic mechanical analysis. Further it was observed that the glass transition of the butadiene-rich phase is mainly determined by the S/B composition of the statistical copolymer block as confirmed by the Fox-equation. The alteration of the glass transition of the PS-rich phase and the observed PS-softening with raise in styrene content might be correlated to the increasing interphase width due to the enhanced miscibility as shown by calculations based on a simple model for diblock copolymers. Tensile testing revealed a transition from ductile to semi-ductile to brittle behaviour that strongly depends on the styrene content in the S/B middle block, chain architecture and the resulting morphology. Block copolymers (BCPs) with lamellar structure exhibited ductile behaviour with extensive strain hardening, whereas BCPs forming segregated structures without long range order were semi-ductile or brittle depending on the type of block architecture and on the hard-phase content. The transition in the mechanical behaviour was confirmed by fracture mechanical investigations based on the essential work of fracture approach and SEM-characterizations. (C) 2012 Elsevier Ltd. All rights reserved.