Mixtures of polystyrene (PS) or poly(methyl methacrylate) (PMMA) homopolymers with symmetric P (S-6-MMA) diblock copolymers were investigated by neutron reflectivity. In a thin-film geometry, these mixtures form alternating lamellar microdomains oriented parallel to the substrate surface. By adding perdeuterated homopolymer to unlabeled copolymer, the spatial distribution of the homopolymer was characterized quantitatively. When the molecular weight of the homopolymer is comparable to the block molecular weight, the homopolymer is confined to the corresponding copolymer domain, with a distribution which peaks at the center of the domain. The results are shown to be in excellent agreement with mean-field predictions. With decreasing molecular weight, the homopolymer is more uniformly distributed within the domain. When the molecular weight of the homopolymer is much larger than the block molecular weight, the homopolymer is excluded from the lamellar microdomains, but does not interfere with the preferred lamellar orientation and is incorporated into the multilayered morphology. © 1992, American Chemical Society. All rights reserved.