The miscibility behavior of reactive ternary blends, a model for reactive hot melt polyurethane adhesives, has been characterized. The model system is composed of polyether and polyester prepolymer blended with a high T g polyacrylate. During prepolymer synthesis, the molecular weight and chemical composition of the polyether and polyester chains were altered. Phase diagrams of the prepolymer were found to differ significantly from those associated with the starting materials. Consistent with our earlier findings, polyether with or without chemical modification plays a crucial role in expanding the region of miscibility. Incorporation of reactive isocyanate groups onto these polymers significantly altered the miscibility behavior of the ternary blends. In contrast, modified polyesters did not improve blend miscibility. These changes in specific interchain interaction were offset by the increase in molecular weight of both components. The competitive influence of polyether and polyester miscibility behavior was analyzed in a quantitative manner. Balancing the polyether and polyester reaction chemistry is critical for controlling the melt miscibility of these ternary blend systems.