Blends containing styrene--butadiene diblock copolymer (.50 wt % styrene content) and polystyrene of various molecular weights are studied by light scattering, transmission electron microscopy, and small-angle X-ray scattering. The solubility of polystyrene in the styrene domain of the block copolymer is governed by the ratio of the homopolymer molecular weight to the block molecular weight. A finite solubility limit exists when this ratio exceeds approximately 1. The lamellar repeat period increases linearly as more polystyrene is added, but the butadiene layer thickness remains constant, signifying that the average interfacial area occupied by a copolymer junction point does not change with added polystyrene. This contrasts to the case found by others that the average area per junction point increases when the added homopolymer is smaller than the block size. Small-angle X-ray scattering patterns obtained from samples having lamellar morphology are described by an idealized model in which layers of styrene and butadiene of randomly varying thicknesses with a diffuse interface between them are stacked parallel.