AbstractNew block copolymers of polystyrene with polyesters have been synthesized and investigated as compatibilizing agents. The addition of block copolymers in immiscible blends of polybutylene terephthalate and (i) polystyrene, (ii) polystyrene/poly(2,6‐dimethyl 1,4‐phenylene oxide) as well as (iii) high impact polystyrene drastically reduced the dimensions of the dispersed phase and stabilized the phase growth with annealing time. Stress‐strain curves for the binary blends indicate brittle behavior. The addition of more than 10 wt% of block copolymers transforms their brittle characteristics to ductile beh