The deformation behavior of blends consisting of a styrene-butadiene star block copolymer and a polystyrene homopolymer was studied by high-voltage electron microscopy with a tensile device. The mechanical properties and micromechanical deformation mechanisms in the star block copolymer/polystyrene blends were directly influenced by their morphology. Although the pure block copolymer deformed in a very unequal manner (because of a thin-layer-yielding mechanism) and revealed no local deformation zones, a transition to the formation of crazelike zones was observed in the blends. This transition in the deformation mechanisms was correlated to the abrupt change in the macroscopic strain at break of the injection-molded specimens. At lower contents of added polystyrene, a craze-stopping mechanism was observed, whereas the blends with higher polystyrene contents demonstrated crazing like that in pure polystyrene. (C) 2003 Wiley Periodicals, Inc.