Morphology and deformation behaviour of binary blends comprising styrene-block-butadiene-block-styrene 1 triblock copolymers having complementary composition, one with 75% styrene and another one with 28% styrene, were investigated by tensile testing and different microscopic techniques such as SEM, TEM and AFM. On the fracture surfaces of the binary blends, two distinct regions of deformation structures, representative of each of the block copolymers, appear indicating a macrophase separation of the blend components. The latter was confirmed by the TEM investigation of the bulk binary blend sample. The experimental results show that besides the overall blend composition, asymmetric architecture of one of the components can significantly modify the phase behaviour of the binary block copolymer blends and hence result in macrophase separation. A sudden shift in the mechanical properties occurs at the composition when the matrix of blend is changed.