The self-assembly of binary mixtures of poly(styrene-block-isoprene) was investigated by small-angle X-ray scattering and transmission electron microscopy. The copolymers investigated had nearly equal compositions of styrene and isoprene but different molecular weights such that each copolymer by itself forms lamellar microdomains in the segregation limit. The mixtures, however, underwent macroscopic phase separation into domains rich in large and small molecular weight copolymers, both forming the lamellar microdomain in the segregation limit. interesting commensurations were formed at the macroscopic interface of the two domains composed of thick and thin lamellae. A possible interpretation of the macrophase separation was proposed on the basis of the macrophase separation induced by the microphase separation.