The thin film phase behavior of ternary blends consisting of symmetric poly(styrene) (PS)-b-poly(dimethylsiloxane)(PDMS), PS, and PDMS was investigated using X-ray reflectivity (XRR) and atomic force microscopy (AFM). This system is strongly segregated, and the homopolymers are approximately the same length as the corresponding blocks of the copolymer. The XRR and AFM data are used to quantify changes in domain spacing (L) and morphology evolution with increasing homopolymer content (phi(H)). In 100 nm thick films, from phi(H) = 0 to 0.20, the system maintains a perfect parallel lamellar structure and domains swell as predicted based on theory; however, from phi(H) = 0.30 to 0.50, a morphology transition to a "dot pattern" morphology (tentatively identified as perforated lamellae) and mixed morphologies were observed before macrophase separation. In thicker films, dot patterns were observed for a broad range of phi(H) before macrophase separation. The absence of the bicontinuous microemulsion phase reported for bulk blends and thin films of perpendicular lamellae and the presence of dot patterns/perforated lamellae are attributed to preferential migration of the PDMS homopolymer to the wetting layers located at the substrate and free air interfaces, which leads to an asymmetric composition within the film and morphology transition. (c) 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018, 56, 1443-1451