The phase behaviors of binary blends of poly(styrene-b-butadiene) block copolymers were investigated by a small-angle X-ray scattering technique. The blends were composed of weakly segregated one in a random micellar phase and the other in a cylindrical phase with similar molecular weights and complementary volume fractions. Morphologies, domain spacings, and order-disorder transition temperatures of the blends indicated that the junctions of the constituent block copolymers share the interface at low temperatures. The domain spacing decreased as temperature increased in a blend with a small amount of the weakly segregated block copolymer. In the cases of the blends with a large amount of the weakly segregated constituent, domain spacing increased with increasing temperature. These results implied that some of the weakly segregated block copolymer moved from the interface to one microdomain at higher temperatures. (c) 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014, 52, 470-476