This paper deals with immiscible blends of poly(ethylene terephthalate)with polycarbonate obtained by melt mixing. Miscibility of the polyester blends is influenced by transesterification reactions, that are catalyzed either by catalyst residues in the polyesters or by catalysts added on purpose in the blend. These reactions convert the initial homopolymers into block and even random copolymers and affect both the miscibility of the system and the adhesion between the phases. The effect of catalysts and stabilizers on the morphology of PET/PC 50/50 blends was investigated using dynamic mechanical thermal analysis, rheology, microscopy and tensile tests. PET/PC 50/50 containing 0.05 wt-% of lanthanide acetyl acetonate exhibit a irreversible transition occurring at temperature higher than the glass transitions of PET and PC. This transition induces a large increase of the shear modulus and it was attributed to the formation of a third phase in the blend.