High density polyethylene (PEHD) and isotactic polypropylene (IPP) blends exhibit a maximum in impact strength at a specific mixing ratio. This effect is even more pronounced if a compatibilizer is added. An explanation was found by analyzing the morphology. From scanning force microscopy results on samples crystallized from the melt it can be concluded that mixing of iPP with PEHD causes a decrease of iPP spherulite size as compared to pure iPP. After processing such an extrusion and quenching no spherulites but an excellent distribution of nucleation centres was found by transmission electron microscopy, indicating an increase of the total interphasial volume within the blend material, which in turn improves the impact strength. On the microscopic level the energetically most favourable interphasial molecular arrangements are obtained by molecular dynamics calculations.