Tensile, dynamic mechanical, thermal properties and morphology features of poly(ethylene terephthalate) (PET) blends with the acrylonitrile-butadiene-styrene (ABS) terpolymer were examined at up to 25 wt% content of ABS. Both unmodified and maleic anhydride grafted ABS (ABS-g-MA) were used. PET/ABS blends quenched from the melt and tested shortly after, show good mechanical properties-the result of component miscibility in the melt predicted by the theory and providing temporal component adhesion in the solid state. After storage, properties deteriorate due to ABS particle debonding and a polymer alloy is obtained with poor tensile properties. This property deterioration is not observed in PET/ABS-g-MA blends due to the grafting of the modified ABS particles onto the PET matrix. These findings are supported by the optical and mainly by the scanning electron microscopy examination of cryofractured and etched blends.