The effect of 12 different potential compatibilizing agents on the interfacial tension of polyethylene (PE) and polyethylene terephthalate (PET) is described. A wide range of compatibilizing agents based upon ethylene and a second comonomer were devised and used. They were divided into three categories. The first category (I) includes block polymers. The second category (II) includes random copolymers. The third category (III) includes reactive copolymers that should form graft copolymers during mixing with PET. The addition of various potential compatibilizing agents to the PE/PET binary system results in lowered interfacial tension. The efficiency of the different compatibilizing agents in reducing the interfacial tension is discussed.