Abstract The effectiveness of the sodium ionomer of poly(ethylene- co-methacrylic acid) as a compatibilizer for poly(ethylene terephthalate)/linear low density polyethylene (PET/LLDPE) was investigated at various PET/LLDPE ratios and ionomer contents. The techniques used were optical and electron microscopy, dynamic mechanical spectroscopy, differential scanning calorimetry and tensile tests. The tensile properties were characteristic of mechanical compatibilization since all other techniques indicated a phase-separated system. The PET/LLDPE ratio influenced the degree of compatibilization obtained. Using moderate levels of the compatibilizer (ca. 10 wt%), best properties were obtained at increased PET levels. On the basis of previous findings on the compatibility of PET/ionomer blends it is proposed that compatibilization should be attributed to strong interfacial bonding at the PET/ionomer interphase. An attempt to predict ternary properties from binary blend data using the appropriate theory of mixtures was moderately successful.