We examined the effect of the addition of homopolymers tetramethylbisphenol A polycarbonate, TMPC, and poly(norbornene-methyl-d(3)-carboxylate), NBMC, on the interlamellar spacing and the phase stability of symmetric poly(styrene-b-methyl methacrylate) diblock copolymer thin films on silicon substrates. The films were of thickness h, L < h < 5L, where L is the interlamellar spacing of the microphase separated PS-b-PMMA diblock copolymer. The homopolymers considered had degrees of polymerization, N-TMPC and N-NBMC, comparable to one-half of the degree of polymerization of the diblock. In the TMPC/diblock blend, for low TMPC homopolymer concentrations, phi(TMPC) < 0.2, the homopolymer chains were localized in the middle of the PS micro-ordered domains, and the interlamellar spacing increased as L/(1 - phi(TMPC)). For phi(TMPC) > 0.2 the morphology of the diblock copolymer changed to accommodate higher volume fractions of TMPC. This behavior is contrasted with earlier observations in the PS-b-PMMA/PS homopolymer system where this diblock accommodated considerably higher PS homopolymer volume fractions while maintaining a lamellar phase. In the NBMC/diblock system the interlamellar spacing increased as L/(1 - phi(NBMC)) for homopolymer concentrations up to phi(NBMC) approximate to 0.05. For phi(NBMC) > 0.05, the NBMC formed a pure layer on the substrate, with the diblock maintaining its microphase separated structure on this layer. These results are discussed in light of mean field theory and in terms of the effect of the interfacial constraints on the phase behavior.