Blends of 75/25 poly(ethylene terephthalate) (PET)/high density polyethylene (HDPE) containing poly(ethylene-co-methacrylic acid) partially neutralized with lithium (PEMA-Li) were obtained by direct injection molding in an attempt (i) to ameliorate the poor performance of the binary blend and (ii) to find the best compatibilizer content. The presence of PEMA-Li caused a nucleation effect on PET, and a decrease in the crystalline content of HDPE. The compatibilizing effect of PEMA-Li was due to the combined effects of interaction at the interface and chemical reactions. The ternary blends showed a complex morphology, with two dispersed HDPE and PEMA-Li phases that contained a small internal dispersed phase, probably of PET. The compatibilizing effect of PEMA-Li was clearly shown by means of an impressive increase in the ductility and to a minor extent in the impact strength. The highest property improvement (ductility increase 1450%) appeared upon the addition of 45% PEMA-Li with respect to the HDPE phase, but taking into account the recycling interest, the ternary blend with the addition of roughly 22.5% PEMA-Li appears to be the most attractive.