Mechanical properties of poly(styrene-b-n-butylmethacrylate) diblock copolymers, PS-b-PBMA, with different lengths of the polystyrene block were investigated. The copolymers display a composition range where the tensile strength of the block copolymers exceeds the values of the corresponding homopolymers. At a PS-content of 74 vol% not only the tensile strength but also the strain at break is higher than that of pure polystyrene. Furthermore, the absorbed energy shows a maximum at a polystyrene content of 29 vol%. This composition reveals a morphology of hexagonally packed PS-cylinders which turns out to be effective for the increase of toughness in PS-b-PBMA diblock copolymers. The improved mechnical properties of PS-b-PBMA diblock copolymers are discussed with respect to a possible correlation and synergism between phase behaviour, morphology, deformation mechanism, and interface formation. © 1998 Elsevier Science Ltd.