The effect of homopolymer (hP) addition on the structure formation in lamellar amorphous block copolymers (BCP) with narrow- and broad-molecular weight distribution (MWD) was studied using small-angle X-ray scattering and transmission electron microscopy. The systems in our study consist of blends of a poly(styrene-b-methyl acrylate) copolymer with block-selective broad MWD of the poly(methyl acrylate) domain as well as polystyrene and poly(methyl acrylate) hPs with molecular weight less than the corresponding block of the copolymer. Homopolymer addition to the broad MWD domain of the BCP is found to induce structural changes similar to narrow MWD BCP/hP blend systems. Conversely, addition of hP to the narrow MWD domain is found to induce a more pronounced expansion of lamellar domains due to the segregation of the hP to the center region within the host copolymer domain. With increasing hP concentration, the formation of a stable two-phase regime with coexisting lamellar/gyroid microphases is observed that is bounded by uniform lamellar phase regimes that differ in the distribution of hP within the corresponding narrow MWD block domain. The segregation of low-molecular weight hP to the center region of the narrowdisperse domains of a broad MWD BCP is rationalized as a consequence of the more stretched chain conformations within the narrowdisperse block that are implied by the presence of a disperse adjacent copolymer domain. The increase of chain stretching reduces the capacity of the narrowdisperse block to solubilize hP additives and thus provides a driving force for the segregation of hP chains to the center of the host copolymer domain. (C) 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 50: 106116, 2012