Lamellar domain spacings (D) of blends of a styrene-2-vinylpyridine (SP) diblock copolymer and homopolymers corresponding to the block polymers (S and/or P), of which molecular weights are lower than that of the block polymer, were examined by small-angle X-ray scattering (SAXS), and the dimensions of block chains in the lamellae were investigated by small-angle neutron scattering (SANS). The D’s for SP/S/P ternary and SP/S (or SP/P) binary blends increase with increasing the molecular weight of the homopolymers, M-H, as well as the volume fraction of the homopolymers, Phi(H), in the corresponding lamella. The variations of the D’s with Phi(H) for the blends with homopolymers having different M-H’s reveal that homopolymers with higher M-H are localized at the center of the corresponding lamella, while those with lower M-H are distributed throughout the lamella. Moreover, the comparison between the D’s for ternary and binary blends with homopolymers having equal M-H’s implies that the homopolymers in binary blends are more localized than those in ternary blends. In these ternary and binary blends the radii of gyration of styrene block chains along the direction parallel to lamellar interface are not much different from that for a pure diblock copolymer irrespective of Phi(H) as well as M-H.