The influence of molecular architecture on the mechanical properties of styrene/butadiene block copolymers was investigated by means of the microhardness technique. It was found that the microhardness of the styrene/butadiene block copolymers is dictated by the nature of microphase separated morphology. In contrast to polymer blends and random copolymers, in which the microhardness generally follows the additivity rule, the behavior of the investigated block copolymers was found to significantly deviate depending on their molecular architecture. The glass-transition temperature of the polystyrene phase (Tg-PS), which practically remained constant and that of the polybutadiene phase (Tg-PB), which varied with the change in the block copolymer architecture, apparently do not influence the microhardness values of the block copolymers. (C) 2003 Wiley Periodicals, Inc.