The effect of composition distribution on microphase-separated structures of AB diblock and BAB triblock copolymers was investigated at the molecular level by measurements of neutron reflectivity. Monodispersed three poly(styrene)-d8-block-poly(2-vinylpyridine) (DP) and three labeled triblock copolymers (PDP) and unlabeled counterparts were synthesized by a living anionic polymerization process designed to yield constant molecular weight with different volume fractions. The selective labeling method employs the blending of copolymers for a segmental distribution study while the labeled polymers are mixed for an interfacial study. Scattering length density profiles were observed by neutron reflectivity measurements for blended thin films produced by spin-coating and successive thermal annealing. It was confirmed that the blended thin films exhibit simple alternating lamellar structures, an observation consistent with the observed structures of bulk specimens. The localization phenomenon of longer and short...