Poly(ethylene terephthalate) (PET)/poly(butylene terephthalate) (PET) blends (xUPET/(1-x)PBT) characterized by amorphous initial structure are prepared by ultraquenching of the melt between two metal rolls rotating in a liquid nitrogen bath. The thermal behaviour, miscibility, crystallization and mechanical properties of PET/PET blends are investigated after three years of storage at -15 degrees C. The physical properties of as-quenched samples change significantly after a long period of storage at low temperature. In contrast to the results for the as-quenched blends, a single glass transition temperature (T-g), intermediate between those of the pure components, is observed at each composition after a long period of low temperature relaxation. The dependence of T-g on the composition can be described by the Fox equation. On the basis of the thermal properties of PET/PET blends, the conclusion drawn is that they are miscible in the amorphous state. Both components crystallize simultaneously at all compositions of the quenched blends. Each component forms its own crystal phase and the presence of the other component does not disturb, and even enhances, the crystallization process. The observed positive synergistic effect in mechanical properties of PET/PET blends is related to their amorphous miscibility.