The morphological behavior of binary mixtures containing poly(styrene-b-2-vinylpyridine) (PS-b-PVP) diblock copolymer and poly(styrene-r-2-vinylpyridine) (PS-r-PVP) random copolymer was investigated as a function of the molecular weight ratio of PS-b-PVP and PS-r-PVP (R), the PS fraction in PS-r-PVP, and the concentration of PS-r-PVP in the blends (ϕr). When R was high, the addition of symmetric PS-r-PVP caused lateral expansion of microdomains and reduced the interdomain distance of the blend, indicating localization of PS-r-PVP at the PS-b-PVP interface. At high ϕr, packing constraints prevented all PS-r-PVP from assembling at the PS-b-PVP interface, which induced macrophase separation and formed a coexisting morphology composed of ordered polymer phase and random copolymer-rich regimes. Reducing the R value reduced the amount of PS-r-PVP that could be assembled at the PS-b-PVP interface, and macrophase separation occurred at a low PS-r-PVP content. When asymmetric PS-r-PVP was introduced into PS-b-PVP, PS-r-PVP was located in the preferred domain of PS-b-PVP because of the favorable interaction of PS-r-PVP with the particular domain. The enthalpically driven self-assembly rendered to swell the preferred domain and increased the interfacial curvature that, in turn, induced an order-order transition.