A 50/50 (weight ratio (38/62 mole ratio referred to repeating units)) blend of poly(butylene terephthalate) (PBT) and polyarylate (PAr), was studied by means of thermal, solubility, X-ray and nuclear magnetic resonance techniques after annealing procedures that enable transesterification. Prolonged thermal treatment at 290 degrees C 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 at the relatively low temperature of 140 degrees C results in the reappearance of melting endotherms in the differential scanning calorimetry curves. This effect is explained by crystallization-induced sequential reordering from random to block copolymer by means of transreactions. In that way a PBT/PAr blend was shown to be another polymer system, along with poly(ethylene terephthalate) (PET)/polycarbonate (PC) and PET/PAr blends, in which the entire cycle is realized, from two homopolymers via a block- and random copolymer to a block copolymer. The unusually low temperature at which the crystallization-induced sequential reordering to block polymers takes place is explained by the miscibility of PET and PAr which enables transreactions to take place in the bulk.