Ordered structures of binary mixtures of poly(styrene-block-isoprene) diblock copolymers (SI) were studied with transmission electron microscopy and small-angle X-ray scattering. In the case when f. congruent-to f(beta) congruent-to 0.8 or f(alpha) congruent-to 1 - f(beta) congruent-to 0.7 and r = N(alpha)/N(beta) less-than-or-equal-to 2, the two copolymers, designated alpha and beta, are found to be mixed at a molecular level for all blending compositions, resulting in a single domain morphology (i.e., spherical microdomains packed in a cubic lattice, hexagonally packed cylindrical microdomains, or alternating lamellar microdomains). Here f(K) and N(K) are the volume fraction of one of the block chains (e.g., polystyrene) and the total degree of polymerization of the Kth copolymer (K = alpha or beta, N(alpha) > N(beta)). Even in the case when f(alpha) congruent-to f(beta) = 0.8, if r greater-than-or-equal-to 7, alpha and beta are found to be partially miscible. They phase-separate into large spheres rich in a and small spheres composed of almost pure beta for the composition (PHI(alpha)) of a component satisfying PHI(alpha) less-than-or-equal-to PHI(alpha,C) congruent-to 0.8. The phase separation was proposed to result from the "fluctuation-induced segregation effect". On the other hand, in the case when f(alpha) congruent-to 1 - f(beta) = 0.8 (or 0.2 and r congruent-to 1.2), the mixtures are found to form a modulated (or superlattice) structure having a spatial arrangement of alpha-rich and beta-rich domains with a characteristic length scale of microns. Each domain rich in alpha or beta has the microdomain structure characteristic of the copolymers alpha or beta with a characteristic length scale of nanometers. The superlattice structure was proposed to originate from the macrophase separation between alpha and beta and subsequent microphase separation induced in the regions rich in alpha and beta copolymers.