Graft copolymers and coionomeric mixtures produced by mixing a phenoxy polymer (polyhydroxyether of bisphenol A) respectively with ethylene-propylene copolymers, containing grafted maleic anhydride groups (EP-g-MA), and the sodium ionomers of the terpolymers of ethylene, an alkyl acrylate and a carboxylic acid, the ionomeric character of which being enhanced by the addition of either sodium benzoate (weak base) or sodium ethoxide (strong base), were used as compatibilizers for blends of PET and HDPE in different ratios and with varied molecular weight of the HDPE component. The preparation of the compatibilizer and the production of the blends were carried out on a twin-screw extruder. Both types of compatibilizers acted as effective surfactants for the dispersion of the two polymers, at all weight ratios, and the blends exhibited a substantial increase in melt viscosity as a result of the addition of the compatibilizer. Only the coionomeric compatibilizers, however, produced substantial improvements in both tensile strength and elongation at break, irrespective of the ratio of the two polymers in the blends. This was attributed to the strong nucleating effects on the crystallization of the polymer components and to a strong interfacial adhesion between the two polymer phases.