The tensile properties of blends based on syndiotactic polypropylene (sPP) and high-density polyethylene (HDPE) have been studied. In order to understand the unexpected decrease in ductility, the crystallization behavior of these blends was characterized by transmission electron microscopy and in-situ Raman spectroscopy. For all investigated blends demixing occurs, and results for low HDPE concentrations in an island-like morphology of HDPE domains embedded in a continuous sPP matrix. For all investigated sPP/HDPE blends the start temperature of the sPP crystallization significantly increases with increasing HDPE content.