In attempt to enhance the compatibility of PET/LDPE blends by using a proper functionalized polymer as third component, diethyl maleate (DEM)-functionalized ultralow density poly(ethylene) (ULDPE-g-DEM) and styrene-b-(ethylene-co-1-butene)-b-styrene triblock copolymer (SEBS-g-DEM) were prepared by radical functionalization in the melt. Immiscible PET/LDPE blends having compositions of 70/30 and 80/20 by weight were then extruded in the presence of 1-10% by weight of ULDPE-g-DEM and SEBS-g-DEM as compatibilizer precursors and ZnO (0.3% by weight) as transesterification catalyst. In both cases, evidences about the occurring of compatibilization between the two immiscible phases, thanks to the studied reactive processes, were obtained. Moreover, the phase distribution and particle size of blends were deeply investigated. Completely different kinds of phase morphology were achieved, as ULDPE-g-DEM stabilized a dispersed phase morphology, whereas SEBS-g-DEM favored the development of a cocontinuous phase morphology. The observed differences are tentatively explained on the basis of reactivity and physical features of polymers.