The final properties of recycled polymers greatly depend on the presence of impurities, which can be detected by differential scanning calorimetry. For the purpose of the present study a total of five high impact polystyrene (HIPS)/polypropylene (PP) blends (100/0; 97.5/2.5; 95/5; 92.5/7.5; 90/10) were prepared and injected at a temperature in the 220-250 degrees C range. The subsequent mechanical characterization indicates a reduction of tensile strength, elongation at break and impact strength. Scanning Electron Microscopy (SEM) was used in the morphological analysis of the different blends. Finally the variations in the flow measurements and glass transition temperature were analysed using melt flow index (MFI) and differential scanning calorimetry (DSC), respectively. The presence of impurities may have a negative effect on the mechanical properties of the material, but may improve material performance during processing.