The morphological behavior of a series of binary mixtures of polystyrene-block-polyisoprene (SI) diblock copolymers has been investigated by small-angle X-ray scattering and transmission electron microscopy. These mixtures consist of a given asymmetric SI diblock, coded as, which itself exhibits a morphology of polystyrene spheres on a body-centered-cubic lattice in a polyisoprene matrix and a symmetric SI diblock of lower molecular weight. This paper addresses the effect of varying the molecular weight of the symmetric diblock on the phase diagram of the binary blend. Three different short symmetric diblocks, coded s(1), s(2), and s(3), have been used. Their molecular weights are such that both s(1) and s(2) are in an ordered state at room temperature, exhibiting the lamellar morpology, while the shortest one, s(3), is in a disordered state. We show that over the whole composition range these binary mixtures do not undergo macrophase separation. The two types of chains self-organize in either a single ordered phase or a single disordered phase. We show that the phase diagram does not depend only on the overall volume fraction of both chemical species in the system, i.e., polystyrene and polyisoprene, but is also strongly affected by the ratio between the chain lengths of the asymmetric diblock and the symmetric diblock, r = N-as/N-s1, where N-as and N-si are the degree of polymerizations of the asymmetric and symmetric diblock copolymers s(i) (i = i, 2, and 3), respectively. The effect of this parameter is a shift of the phase boundaries between neighboring morphologies. Therefore, blending diblocks of different lengths and compositions enables one to alter the relationship between thermodynamically stable morphology and volume fraction, which is not possible in single AB diblock systems alone. By analogy with surfactant systems and recent theoretical works of Shi and Noolandi, it can be described as a "cosurfactant effect": a small amount of a short symmetric SI effectively changes curvature of the domains formed by a long asymmetric SI.