Asymmetric styrene/butacliene block copolymers and their blends with polystyrene homopolymer are studied, using transmission electron microscopy and scanning force microscopy, to explore the influence of phase morphology on the microindentation hardness and the nano-mechanical deformation mechanisms. In contrast to polymer blends and random copolymers, in which microhardness generally follows the additivity law, the behaviour of the investigated block copolymer systems is found to significantly deviate from the hardness additivity behaviour. Owing to the modified architecture, the asymmetric star block copolymer having 74 vol% polystyrene possesses a lamellar morphology, which partly explains the observed low hardness values. An additional explanation is found in the large plastic homogeneous deformation of the polystyrene (PS) lamellae by means of a new micromechanical mechanism called thin layer yielding. In the blends of a star block copolymer with polystyrene homopolymer, a rapid change in the micromechanical deformation behaviour is found to cause a shift in the observed microhardness to larger values. A major conclusion of our investigations is that microhardness of the triblock and star block copolymers can only be explained in the light of the morphology and nano-mechanical deformation mechanisms involved. (C) 2003 Kluwer Academic Publishers.