In this study, three PS/HDPE blend compositions (10, 30 and 70% PS) were processed in a twin-screw extruder via three procedures. In the first case, pellets of HDPE and PS were added in the hopper of the twin-screw extruder and were blended exclusively on the twin-screw in the classical manner. In the second case HDPE pellets were fed at the hopper of the twin-screw extruder and the PS was added as a melt using a single screw extruder connected midway to the twin-screw. In the third case the PS melt was added even farther downstream with a residence time in the extruder of approximately 25 s. The results show that at all three compositions, the dispersed phase diameter and size distribution are independent of the mixing procedures. These results clearly illustrate that the final blend morphology does not depend on the melting or softening step, but is determined predominantly in the melt slate and this even at high compositions. The morphology is developed very rapidly under these dynamic conditions and it is essentially the last flow environment experienced by the blend melt that determines its final morphology.