An equimolar blend of poly(ethy1ene terephthalate) (PET)' and bisphenol-A-polycarbonate (PC)d is studied by dynamic-mechanical thermal analysis (DMTA) and X-ray scattering after thermal treatment that enables transesterification. As demonstrated by wide-angle X-ray scattering (WAXS) measurements, prolonged thermal treatment at 280ยฐC gives rise to a copolymer that no longer reveals melting or crystallization. In accordance with previous reports, this effect is attributed to the formation of a random copolymer. Additional annealing of such samples below the melting temperature of PET results in restoration of the crystallization ability. This effect is explained by crystallization-induced sequential reordering from random to block copolymer by means of transreactions which closes the cycle of transformations from two homopolymers via block-and random copolymer back to a block copolymer. The behavior of the amorphous phases is studied by means of DMTA demonstrating that their glass transition temperatures Tg's vary in accordance with the crystallinity changes. The random copolymer is characterized by a more or less homogeneous amorphous phase. In contrast to this, the mechanical mixture and the two block copolymers (the initial and that with the restored blocky structure) show DMTA peaks of two amorphous phases, clearly separated and with distinct individual Tg's. Viscosity measurements also demonstrate that the random copolymer significantly differs in its viscosity as compared to all other samples. These results represent a further evidence for the effect of block restoration via crystallization-induced sequential reordering.