Morphological investigations of solvent-cast polymer blend films, containing dilute dispersions of poly(methyl methacrylate-block-styrene) (PMMA-b-PS) in a matrix of either poly(methyl methacrylate) (PMMA) or poly(vinyl chloride) (PVC) are described. PMMA and PVC species having molecular weights ranging from 25 000 to 400 000 and from 120 000 to 500 000, respectively, were blended with PMMA-b-PS copolymers of molecular weight 20 000, 65 000, 283 000, and 680 000 at a composition of 80:20 w/w. The PMMA/PMMA-b-PS blends exhibit a variety of micellar morphologies (viz. spherical, cylindrical, vesicular, and lamellar structures) which can be systematically modified by varying the molecular weight ratio of homopolymer PMMA to block segment PMMA, lambda = M(H)/M(A). For blends in which lambda << 1, spherical micelles of uniform size are formed. On increasing lambda, the micellar size increases initially, and then a transition from spherical to cylindrical to vesicular and lamellar morphology occurs, provided lambda remains less than 1. Further increase of lambda to values larger than 1 produces morphologies in which macrophase separation accompanies microphase formation. The PVC/PMMA-b-PS blends exhibit microphase formation, even at lambda > 1, due to the strong exothermic interaction between PVC and PMMA. The morphology of this blend system was studied as a function of lambda and of the product Nchi PVC-PS, where chi(PVC-PS) is the PVC-PS Flory-Huggins interaction parameter, and N = NPVCNPS/(N-PVC(1/2) + N-PS(1/2))(2). Macrophase formation is observed when lambda >> 1 and also occurs when lambda < 1, if N chi(PVC-PS) becomes very large. Microphase morphology in the PVC/PMMA-b-PS blends is insensitive to the value of lambda because of the dominant role played by the exothermic interaction in determining coronal swelling of the block copolymer.