The comparison of the degree of crystallinity and the micromechanical properties in the blends of recycled amorphous poly(ethylene terephthalate) (PET) with isotactic polypropylene (iPP) and high-density polyethylene (HDPE) with a compatibilizer in different proportions is reported. The physical study of the composites of the compatibilized blends and clay is also discussed. The analysis, performed by means of wide-angle X-ray scattering and differential scanning calorimetry techniques, permits us to describe, at microscale level, the role of the compatibilizer on the structure and microhardness of the polymer blends that we studied. The results reveal that PET was incompatible with both iPP and HDPE. However, the presence of the compatibilizer, a styrene ethylene/butylene styrene block copolymer grafted with maleic anhydride, allowed the compatibilization of these polymers. In the PET/iPP blends, the clay seemed to have a nucleating effect on the iPP and also induced a hardness increase in the compatibilized blends. On the other hand, in case of PET/HDPE, the crystallinity of these samples (pure blends, blends with compatibilizer, and blends with compatibilizer plus clay) only depended on their composition. Similarly to the PET/iPP blends, the addition of clay induced an increase in the hardness of the compatibilized blends. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 118: 1278-1287, 2010