The correlation between morphology, mechanical properties, and micromechanical deformation behavior of the blends consisting of an asymmetric styrene/butadiene star block copolymer (ST2-S74, total styrene volume content Phi(PS) = 0.74) and general-purpose polystyrene (GPPS) was investigated using transmission electron microscopy and uniaxial tensile testing. Addition of 20 wt % of GPPS to the block copolymer resulted in a drastic reduction in strain at break, indicating the existence of critical PS lamella thickness D-c Above D-c lamellar block copolymers displayed a transition from ductile to brittle behavior, substantiating the mechanism of thin layer yielding proposed for lamellar star block copolymers. The blends showed a variety of deformation structures ranging from classical crazelike zones to those with internal shearlike components. (C) 2004 Wiley Periodicals, Inc.