The formation of a microphase and a macrophase in solvent-cast homopolymer/block copolymer blends, where the homopolymer is exothermically miscible with one of the block copolymer segments, is demonstrated to depend on the molecular weights of the homopolymer and the incompatible segment of the block copolymer, in spite of the exothermic mixing. Binary blends of poly(vinyl chloride) (PVC) with poly(n-butyl methacrylate-b-styrene) (PnBMA-b-PS) block copolymer, solvent cast from methyl ethyl ketone, were investigated. By varying the molecular weight, we vary the thermodynamic product N-chi PVC/PS which controls the phase morphology. Here N = NPVCNPS/(N-PVC(1/2) + N-PS(1/2))(2), where N-PVC and N-PS are the degrees of polymerization of PVC and PS block copolymer segment, respectively, and chi(PVC/PS) is the Flory-Huggins parameter which characterizes the repulsive interaction between PVC and PS. Microphase separation is observed when N-chi PVC/PS is small; and when N-chi PVC/PS is large, macrophase or macrophase-induced microphase separation occurs. The routes of phase formation are discussed using equilateral triangular phase diagrams, and a morphology diagram is constructed for homopolymer/block copolymer blends. Microphase formation is observed even when the molecular weight of the homopolymer is much larger than that of the miscible block copolymer segment, and when the homopolymer is polydisperse. Indeed, since macrophase formation occurs when N is large, it is clear that low molecular weight block copolymer is necessary to form the microphase when a homopolymer of high molecular weight is present.