The morphology and spatial segmental distribution of constituent polymers, in particular binary mixtures of polystyrene-bloch-polyisoprene (SI) and homopolystyrene, either protonated (HS) or deuterated (DS), have been studied with small angle X-ray scattering (SAXS) and neutron scattering (SANS). The pure block copolymer SI used in this study has a lamellar microdomain morphology. The molecular weights of HS and DS are similar to each other and to that of the polystyrene block (PS) in SI. The SAXS results obtained for the mixtures of SI and HS show that (i) HS is solubilized in the PS microdomains and (ii) the polyisopropene lamellae have a thickness independent of the volume fraction of HS (Phi(HS)), but their interface undulation increases with Phi(HS). These two,findings, in turn, imply that added HS is localized in the middle of the PS microdomains, i.e., between the PS brushes emanating from the opposing lamellar interfaces of SI. This implication was further confirmed by SANS with a deuterium labeling technique; the spatial distribution of the DS segments in the SI/DS mixtures was indicated to be localized in the middle of the PS lamellae with the penetration depth of ca. 11 nm between the DS chains and the PS brush.