AbstractThe separation of phases in defined multiblock (b‐styrene‐bisphenol A carbonate) copolymers was analyzed by small‐angle X‐ray scattering. It was found that these copolymers form typical microdomain structures. For copolymers containing 0.34 and 0.47 wt.‐parts polycarbonate (PC) the microdomains are arranged in the face‐centred cubic macronetwork. The distance between domains decreases with increasing concentration of polycarbonate. The copolymer containing 0.767 PC has lamellar structure. The thickness of the domain‐boundary interphase was quantitatively estimated by analyzing a systematic deviation of the scattering curves at large angle tail from the Porod's law. The thickness of the transition layer decreases with increasing concentration of