The macro- and microphase transitions in binary blends of poly(styrene-b-isoprene) (SI) and poly(styrene-b-ethylenepropylene) (SEP) lamellar diblock copolymers were investigated. by optical and electron microscopy and by rheological analysis. A variety of morphologies were produced as a function of blend composition and the rate of solvent evaporation. Numerous morphologies are possible because of the interaction of macro- and microphase separation. As one example, spherical droplets of SEP whose lamellae comprise concentric spherical shells were produced. At the phase boundaries of this and other morphologies, the polystyrene (S) microdomains associated with each phase were in contact. Additional phase separation occurred within the ordered media. When the molecular weight of SI was small and its mobility high, an SI domain was produced at the center of each spherical SEP droplet, thereby reducing the curvature of the SEP lamellae. When the molecular weight of SI was larger and its mobility lower, individual SI lamellae formed within the ordered SEP-rich phase. The solubility of SI in SEP exceeded that of SEP in SI, because of the relative interaction parameters and molecular weights of the copolymers. SI that was dissolved within SEP localized at the microdomain interface and acted to shield the S and EP segments from one another. The lamellar ordering transition of the lower molecular weight SI diblock was determined theologically. In blends with SEP, the ODT of the SI was increased. This effect was shown to arise from interfacially induced ordering.