Isotactic polypropylene blends with 0-20 vol % thermoplastic elastomers were prepared to study the influence of elastomer particle size on mechanical properties. Polystyrene-bloch-poly( ethene-co-but-l-ene)-block-polystyrene (SEBS) was used as thermoplastic elastomer. SEES particle size, determined by means of transmission electron and atomic force microscopy, was varied by using polypropylene and SEES of different molecular weight. With increasing polypropylene molecular weight and, consequently, melt viscosity and decreasing SEES molecular weight, SEES particle size decreases. Impact strength of pure polypropylene is almost independent of molecular weight, whereas impact strength of polypropylene blends increases strongly with increasing polypropylene molecular weight. The observed sharp brittle-tough transition is caused by micromechanical processes, mostly shear yielding, especially occurring below a critical interparticle distance. The interparticle distance is decreasing with decreasing SEES particle size and increasing volume fraction. If the polypropylene matrix ligament between the SEES particles is thinner than 0.27 mu m, the blends become ductile. Stiffness and yield stress of polypropylene and polypropylene blends increase with increasing polypropylene molecular weight in the same extent, and are consequently only dependent on matrix properties and not on SEES particle size.