The influence of the addn. of poly(Me methacrylate)-block-polystyrene (PMMA-b-PS) and PS-b-poly(Bu acrylate) (PS-b-PnBA) diblock copolymers on the structure and the mech. properties of PMMA/PnBA polymer blends was investigated. In binary polymer blends of PMMA and PS, and PS and PnBA, and in the ternary system the corresponding diblock copolymers caused a significantly higher distribution of the dispersed phase and an appreciable narrowing of the particle size distribution as shown by scanning force microscopy using the pulsed force mode technique. Transmission electron microscopy images proved that in the ternary system the two diblock copolymers did not form their own sep. phase but are arranged as a ring layer at the interface between the two homopolymers PMMA and PnBA. Charpy tests demonstrated that the impact strength of the ternary blends is increased by the two diblock copolymers even at low amts. of the latter. The influence on mech. properties like Young's modulus, yield stress, and tensile strength at break was less marked as strain-stress measurements showed. The major effect seems to be exerted on the elongation at break.